This is a bet made by a player who was not the aggressor in the previous betting round. Specifically, it's when a player who did not make the last aggressive action on the previous street (typically the preflop raiser) leads with a bet on the subsequent street (usually the flop or turn). Donk bets are often perceived as unconventional and can be used for various strategic purposes, such as to take control of the betting, trap opponents, or disrupt the flow of the hand.
The term "donk" (from "donkey") in this context is somewhat pejorative and suggests that the bet is made in an unexpected or non-standard manner.
This bet by Player B is a donkbet because they were not the aggressor before the flop.