Wellspring of life - Compare Proverbs 10:11 note. "the instruction of fools" Not that which they give, but that which they receive. Compare Proverbs 14:24. "Folly" is its own all-sufficient punishment.

Understanding is a well-spring of life - מקור חיים mekor chaiyim; another allusion to the artery that carries the blood from the heart to distribute it to all the extremities of the body.

Understanding [is] a wellspring of life to him that hath it: but the (l) instruction of fools [is] folly.
(l) Either that which the wicked teach others, or else it is folly to teach them who are malicious.

Understanding is a wellspring of life unto him that hath it,.... "The master" or "owner of it" (k). As he only is to whom an understanding is given; for, whatever understanding men may have of natural and civil things, they have none of things spiritual and divine, unless it be given them by Christ. This is no other than the grace of the Spirit of God, who is a spirit of wisdom and revelation in the knowledge of Christ; and this is a well of living water, springing up unto everlasting life, and it issues in it; with the knowledge of Christ, and God in Christ, eternal life is connected, John 4:14; and as this knowledge and understanding of things is communicated by wise and knowing men, they are the means and instruments of the spiritual life of those to whom they minister, and are made useful;
but the instruction of fools is folly; the best instruction they are capable of giving is downright folly, and issues in death.
(k) "domini sui", Pagninus; "domino suo", Mercerus, Gejerus; "dominorum suorum", Michaelis.

As waters to a thirsty land, so is a wise man to his friends and neighbours.

Understanding--or, "discretion," is a constant source of blessing (Proverbs 13:14), benefiting others; but fools' best efforts are folly.

22 A fountain of life is understanding to its possessor;
But the correction of fools is folly.
Oetinger, Bertheau, and others erroneously understand מוּסר of the education which fools bestow upon others; when fools is the subject spoken of, מוּסר is always the education which is bestowed on them, Proverbs 7:22; Proverbs 1:7; cf. Proverbs 5:23; Proverbs 15:5. Also מוסר does not here mean education, disciplina, in the moral sense (Symmachus, ἔννοια; Jerome, doctrina): that which fools gain from education, from training, is folly, for מוסר is the contrast to מקור חיּים, and has thus the meaning of correction or chastisement, Proverbs 15:10, Jeremiah 30:14. And that the fruits of understanding (Proverbs 12:8, cf. שׂכל טוב, fine culture, Proverbs 13:15) represented by מקור חיים (vid., Proverbs 10:11) will accrue to the intelligent themselves, is shown not only by the contrast, but also by the expression: Scaturigo vitae est intellectus praeditorum eo, of those (= to those) who are endowed therewith (The lxx well, τοῖς κεκτημένοις). The man of understanding has in this intellectual possession a fountain of strength, a source of guidance, and a counsel which make his life secure, deepen, and adorn it; while, on the contrary, folly punishes itself by folly (cf. to the form, Proverbs 14:24), for the fool, when he does not come to himself (Psalm 107:17-22), recklessly destroys his own prosperity.

The instruction - Their most grave and serious counsels are foolish.