What does Proverbs 26:3 mean?

Whips and bridles are tools used to train and control animals. If donkeys and horses were rational, and could be talked to, no such measures would be necessary. The point here is not so much about the best way to train livestock. Instead, the meaning is that unreasoning animals receive brute-force corrections. The "fool," meaning a person who ignores godly wisdom (Proverbs 1:7), is unlikely to respond to reason and will suffer as a result (2 Peter 2:12; Jude 1:10). Lessons the wise learn by listening, the God-defying person absorbs only through consequences (Proverbs 14:3; 19:29).

At their best, whips and bridles can be used gently to remind animals of what they've been trained to do. The tug of the bridle, or a light stroke from a whip, might be much less painful than an animal wandering into danger or off the road. Even so, it's no compliment to be compared to barnyard creatures. The point being made is that those who won't listen to godly advice are forced to learn through pain and suffering.

Those who ignore God's revealed truth are described as "fools" (Psalm 14:1). Such persons reject reason when interpreting nature. They are not persuaded by Scripture that God exists and is knowable through Jesus Christ. Therefore, they pursue paths away from God, where eternal punishment lies ahead. A person may be intellectual and brilliant, but if they refuse God's reason and revelation, they are fools bound for misery.