it is when you make a bet that breaks the flow of aggression in the hand. An example would be if someone raises preflop and you call out of position. The normal flow would dictate that you check to him and let him bet on the flop, but betting out before he has the chance to bet is donkbetting. Also if you raise preflop, get called from someone in position, bet the flop, get raised, call the raise, and then bet again on the turn, thats also a donkbet, because the natural play would be to check the turn to him.