When a Pawn reaches the eighth rank, you must turn it into any other piece except a King. That's quite an accomplishment for such a little guy! They are almost always promoted to Queens, since Queens are the most powerful pieces. In chess notation, pawn promotion is written as an equals sign, followed by the piece letter.
White has played b8=Q. Black promoted his Pawn to a Knight, which was a mistake, because the Knight cannot avoid capture after White plays Qf4. Black should have promoted his Pawn to a Queen.