A man of understanding.
v.25 Foolish son. A foolish son brings grief and bitterness on his parents. Motivation for parents to train a child in wisdom. See 10:1 and v.21.
v.26 Impose a fine. It is not good to impose a fine on a righteous man. Corrupt judges are perhaps again in view here. See v.15. The next part of the verse continues the thought by saying it is not good to strike a noble man for his uprightness.
v.27 A cool spirit. A man who restrains his words has knowledge (wisdom). And the one with a cool spirit (coolheaded) has understanding. Restraint in words gives one time to consider what to say.
v.28 The silent fool. Even a fool is considered wise/intelligent when he keeps silent. Silence is evidence of wisdom and a fool is better off to keep silent and not expose his folly. See Jb 13:5.

Read Proverbs 17:25-28