Man, Proverbs really hates folly—it's right up there with adultery, though you could easily see adultery as being but another species of what Proverbs means by "folly." But what is folly? It's connected with a generally unreflective attitude—just babbling on, asserting your own interests and opinions without consideration for anyone else or anything else.
Foolish is destructive largely because it's centered on thoughtless selfishness and not in regard for anything higher. Proverbs sees "the fear of the Lord" as being the basis of wisdom—and fools tend to lack this basis. In a way, that's another definition of folly—any form of human activity that operates without reverence for some greater power (like God or Wisdom) as its basis. Don't do it, says Proverbs.
Questions About Folly and Foolishness
Why are fools foolish? What made them that way? Does Proverbs have any answers?
Does Proverbs think it's possible to teach a fool or a simple person? If not, what's the point of Proverbs? Is it that people who already are inherently wise can become even wiser by reading it?
What makes folly so attractive to fools? If you've read or are reading Ecclesiastes, you can draw on its discussion of folly to help you answer this question.
Why is being quiet associated with being wise and talking too much associated with foolishness? Can there be people who are wise and yet extremely talkative? Why do you think so?