Playin is a slang word for playing. For example: I was just playin around. Also used as an easy hash tag word #playin. The work "playin" is used in may songs and rap songs since it is easier to rhyme with.
Guy 1: How'd it go with those hoes last night homie?
Guy 2: Good, man I was playin' T-Ball so you know I went home and knocked three different girls out of the park.