You play White with a king and two rooks. Checkmate the lone black king within 50 moves to win. This is real chess against a real engine, and it is the easiest of the three mates to learn, because the two rooks can do it without much help from your king.
The trick is the ladder, sometimes called the lawnmower. One rook cuts the black king off along a rank, then the other rook gives check from the next rank, pushing the king back a step. Take turns with the rooks and the king climbs the ladder to the edge, where the last check is mate. If the king comes close enough to attack a rook, slide that rook to the far side of the board and carry on.
Stalemate is a draw, so always leave the black king a square until the mating move. Losing one rook is not the end, because a king and rook can still win, but if the king takes both rooks the game is a draw. The three-fold repetition rule is ignored.