Play for Keeps is a term used in marble games. One type of marble games allow the winning player to keep all of the opponent's marbles that they've won, thus when playing for keeps, it's important to take the game seriously due the possibility of losing all of your marbles.
That term is also used outside of marble games with a similar meaning. The term means that whatever item is being discussed is being taken very seriously. Criminals often say 'We play for keeps' which means that they are not doing what they are doing for enjoyment, and take it very seriously.
Rappers often use this line to mean that they are serious about whatever subject is being discussed. For example, in L'A Capone had a song titled 'Play For Keeps', the whole song was about how he takes the streets seriously.