Is esteemed - Or, "is" (simply). The maxim would imply that silence is in any case good.

Even a fool - He is counted wise as to that particular. He may know that he cannot speak well, and he has sense enough to keep from speaking. He is, as to that particular, a wise fool.
A man may be golden-mouthed and silver-tongued in eloquence; but to know when and where to speak and to be silent, is better than diamonds. But who that thinks he can speak well can refrain from speaking? His tongue has no rest.

Even a fool, when he holdeth his peace, is counted wise,.... Not only one that is sparing of his words, and is really a man of knowledge and understanding; but even a feel, if he is but silent, and does not betray his folly by his words, will be reckoned a wise man by those that do not know him; and, whatever fool he may be in other respects, yet in this he acts the wise part, that he holds his peace and says nothing;
and he that shutteth his lips is esteemed a man of understanding; and keeps them shut, lest he should say anything rashly and hastily; a man that has so much command of himself as not to speak unadvisedly, through the heat of his own passions, and through the provocations of others, will pass for a man that understands himself, and knows how to behave well before others.

Ver. 28 continues the same theme, the value of silence:
Even a fool, when he keeps silence, is counted wise;
When he shutteth his mouth, discreet.
The subj. as well as the pred. of the first line avail for the second. אטם, obturare, occludere, usually of the closing the ear, is here transferred to the mouth. The Hiph. החרישׁ means mutum agere (cf. Arab. khrs, mutum esse), from חרשׁ, which, like κωφός, passes from the meaning surdus to that of mutus (Fl.). The words of Job 13:5, and also those of Alexander: si tacuisses sapiens mansisses, are applicable to fools. An Arab. proverb says, "silence is the covering of the stupid." In the epigrammatical hexameter,
πᾶς τις ἀπαίδευτος φρονιμώτατός ἐστι σιωπῶν,
the word σιωπῶν has the very same syntactical position as these two participles.
(Note: Cf. C. Schultze's Die bibl. Sprichwrter (1860), p. 60f.)