Donk bet
A donk bet is when the player who called a raise before the flop leads out with a bet on the next street instead of checking to the original raiser.
The name comes from the idea that betting into the person who showed strength last round used to be seen as a beginner ("donkey") move. In a typical hand, the player who raised before the flop is expected to bet again after the flop, and everyone else checks to them. A donk bet flips that script: the caller, who is out of position, fires first instead of waiting.
It comes up most often on the flop. Say you call a raise from the big blind, the flop comes, and rather than checking to let the raiser make their continuation bet (c-bet), you bet right into them. That lead is the donk bet. It can show up on the turn or river too, any time the out-of-position caller takes the betting initiative away from the previous aggressor.
For beginners, the honest note is that donk betting is usually a small leak, not a weapon. Most of the time it's better to check and either call or check-raise, because leading out can bloat the pot with a so-so hand and tells your opponent something about your range. It does have its spots, like boards that hit your hand far better than the raiser's, but treat it as a deliberate choice rather than a default. With practice chips you can experiment freely and see how it plays out without any pressure.
Example
You defend your big blind by calling a raise from the button, and the flop comes 9-8-7 with two hearts. Instead of checking to the raiser, you bet about half the pot into them with your 6-5 straight — that lead is a donk bet.