What is before thee - Beware lest dainties tempt thee to excess. Or, "consider diligently who is before thee," the character and temper of the ruler who invites thee.

When thou sittest to eat with a ruler - When invited to the table of thy betters, eat moderately. Do not appear as if half starved at home. Eat not of delicacies to which thou art not accustomed; they are deceitful meat; they please, but they do not profit. They are pleasant to the sight, the taste, and the smell; but they are injurious to health. These are prudential cautions; and should be carefully observed by all who would avoid the conduct of a clown, and desire to pass for a well-bred man.

When thou sittest to eat with a ruler, (a) consider diligently what [is] before thee:
(a) Eat with sobriety.

When thou sittest to eat with a ruler,.... Either a supreme ruler, a king, or a subordinate ruler, a nobleman, a judge, a civil magistrate, a person of honour, dignity, and authority; and to sit at table with such is sometimes allowed, and is always reckoned an honour; this particularly diligent and industrious persons are admitted to, who not only are brought to stand before kings and great persons, but to sit at table with them, and eat food with them. And now the wise man advises such how to behave themselves when this is the case:
consider diligently what is before thee; or, "considering consider" (q): take special notice of the food and drink set upon the table, and consider well which may be most proper and safe to eat and drink of; for though a man may lawfully eat of whatsoever is set before him; every creature of God being good, if it be received with thanksgiving, and sanctified by the word of God and prayer; yet it is a piece of wisdom to make use of that which is most conducive to health, and less ensnaring; and to observe moderation in all, and not indulge to gluttony and drunkenness: and he should consider also who is before him, which sense the words wilt bear; the ruler that has invited him, and sits at the table with him, and take care that he says or does nothing that may give him offence; and also the noble personages that are guests with him, and behave towards them suitably to their rank and dignity; observe their words and conduct, and imitate the same; yea, even he should consider the servants and waiters that attend, lest, behaving in an indecent and disorderly manner, they should report it to his disadvantage to their ruler or others. But how much greater an honour is it to sit at table with the King of kings, and with his princes, and sup with him! when it becomes the saints, who have this honour, to consider what is set before them; the richest dainties, a feast of things, the body and blood of Christ, which should he spiritually discerned by faith; and not the elements of bread and wine only: likewise the persons before whom they are should be considered; Christ, who sits at his table, and the princes of his people with him; and therefore should not feed without fear, and in a disorderly and indecent manner, as the Corinthians are charged, but with all reverence and humility.
(q) "considerando considera", Pagninus, Vatablus, Piscator, Mercerus, Gejerus.

God's restraints of the appetite only say, Do thyself no harm.

(Proverbs. 23:1-35)
Avoid the dangers of gluttony.

Proverbs 22:29, which speaks of a high position near the king, is appropriately followed by a hexastich referring to the slipperiness of the smooth ground of the king's court.
1 When thou sittest to eat with a ruler,
Consider well whom thou hast before thee.
2 And put thy knife to thy throat
If thou art a man of good appetite.
3 Be not lustful after his dainties,
Because it is deceitful food.
The ל of ללחום is that of end: ad cibum capiendum, thus as one invited by him to his table; in prose the expression would be לאכל לחם; לחם, to eat, is poet., Proverbs 4:17; Proverbs 9:5. The fut. תּבין clothes the admonition in the form of a wish or counsel; the infin. intens. בּין makes it urgent: consider well him whom thou hast before thee, viz., that he is not thine equal, but one higher, who can destroy thee as well as be useful to thee. With ושׂמתּ the jussive construction begun by תבין is continued. Zckler and Dchsel, after Ewald and Hitzig, translate incorrectly: thou puttest..., the perf. consec. after an imperf., or, which is the same thing, a fut. meant optatively (e.g., Leviticus 19:18 with לא, and also Leviticus 19:34 without לא) continues the exhortation; to be thus understood, the author ought to have used the expression שׂכּין שׂמתּ and not ושׂמת שׂכין. Rightly Luther: "and put a knife to thy throat," but continuing: "wilt thou preserve thy life," herein caught in the same mistake of the idea with Jerome, the Syr., and Targ., to which נפשׁ here separates itself. שׂכּין (סכּין) (Arab. with the assimilated a sikkı̂n, plur. sekâkı̂n, whence sekâkı̂ni, cutler) designates a knife (R. סך שך, to stick, vid., at Isaiah 9:10). לוע, from לוּע, to devour, is the throat; the word in Aram. signifies only the cheek, while Lagarde seeks to interpret בּלעך infinitively in the sense of (Arab.) bwlw'ak, if thou longest for (from wl'a); but that would make 2b a tautology. The verb לוּע (cf. Arab. l'al', to pant for) shows for the substantive the same primary meaning as glutus from glutire, which was then transferred from the inner organ of swallowing (Kimchi, בית הבליעה, Parchon; הוּשׂט, aesophagus) to the external. "Put a knife to thy throat, is a proverbial expression, like our: the knife stands at his throat; the poet means to say: restrain thy too eager desire by means of the strongest threatening of danger - threaten as it were death to it" (Fleischer). In בּעל נפשׁ, נפשׁ means, as at Proverbs 13:2, desire, and that desire of eating, as at Proverbs 6:30. Rightly Rashi: if thou art greedy with hunger, if thou art a glutton; cf. Sir. 34:12 (31:12), "If thou sittest at a great table, then open not widely thy throat (φάρυγγα), and say not: There is certainly much on it!" The knife thus denotes the restraining and moderating of too good an appetite.
In 3a the punctuation fluctuates between תתאו (Michlol 131a) and תתאו; the latter is found in Cod. 1294, the Erfurt 2 and 3, the Cod. Jaman., and thus it is also to be written at Proverbs 23:6 and Proverbs 24:1; ויתאו, 1-Chronicles 11:17 and Psalm 45:12, Codd. and older Edd. (e.g., Complut. 1517, Ven. 1515, 1521) write with Pathach. מטעמּות, from טעם, signifies savoury dishes, dainties, like (Arab.) dhwâkt, from dhâk (to taste, to relish); cf. sapores, from sapere, in the proverb: the tit-bits of the king burn the lips (vid., Fleischer, Ali's Hundred Proverbs, etc., pp. 71, 104). With והוּא begins, as at Proverbs 3:29, a conditioning clause: since it is, indeed, the bread of deceit (the connection like עד־כּחבים, Proverbs 21:28), food which, as it were, deceives him who eats it, i.e., appears to secure for him the lasting favour of princes, and often enough herein deceives him; cf. the proverb by Burckhardt and Meidani: whoever eats of the sultan's soup burns his lips, even though it may be after a length of time (Fleischer). One must come near to a king, says Calovius, hitting the meaning of the proverb, as to a fire: not too near, lest he be burned; nor too remote, so that he may be warmed therewith.

Ruler - With a great man.