A bet made by an out of position player made into the previous round aggressor is the donk bet.
For example, let’s assume that a player sitting on the button opens the pot by raising, and the big blind calls. If big blind decides to lead on the flop and bets into the preflop aggressor instead of checking, that is a donk bet.
While it was considered a very weak play only made by inexperienced players even several years back, poker solvers proved that this move could be an effective part of poker strategy and is now utilized even by the pros.
That being said, to use this play effectively, you need to balance your game and leave value bets and bluff in both checking and donk betting ranges, making it a difficult task for beginners. If you are not sure how to utilize this properly, you are better off avoiding donk bets altogether.