To "play for keeps" basically means to play rough, sometimes too rough. The players face each other with extreme seriousness and seek to cause damage, and no holds are barred. This is a warning to all "players"--who most times honestly are not playing in any sense of the word--that this is not a joke, and that the victor will claim as much as is possible in the context of said game. "Playing for keeps" can sometimes mean the opponents seek to kill each other in the case of violent combat. Or, at the other end of the spectrum, it could simply mean that in a children's game, the game pieces (cards, coins, marbles, et cetera) won in the contests will actually be kept. No matter what the situation, this phrase carries a warning that the outcome, whatever it may be, is irreversible.