You play White with a king and a rook. Checkmate the lone black king within 50 moves to win. This is real chess against a real engine, and it is harder than the queen version because a rook can't do it alone.
The idea is to use the rook to cut the black king off, then walk your own king up to take squares away, and repeat until the black king is on the edge. Keep your rook defended or far away, because if the king captures it the game is a draw.
Stalemate is a draw too, so always leave the black king a square until the mating move. The three-fold repetition rule is ignored.