Quick answer
Appetite reveals the person. The understanding heart goes looking for knowledge; the fool grazes on nonsense and calls it a meal.
Read both lines
The first line makes learning active. Understanding does not sit waiting to be informed; it "seeks," which is hunting language, and the search is where wisdom starts rather than where it ends. The second line answers with a grazing image: "feeds on foolishness," the mouth taking in whatever is nearest without evaluating it. The contrast is between diet chosen and diet defaulted. Everyone is consuming something, and the verse's quiet question is whether you went after what you take in or simply stood where it was being served.
Related: Proverbs 15:7 · Proverbs 4:17 · Proverbs 18:15
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.