It means I don’t know in short since the newer generations of teens cannot be bothered to type long words. So ,as a result of laziness, they resort to abreviations .
Most commonly used as a shorter version of "i don't know" but in most occasions you don't give a crap about what there saying therefore it becomes" i don't care" because really, you don't.
1: Hey 2: Hey 1: wuu2?
2: Have a guess
1: idk
2: Have a go!
1: no i'm serious idk
2: Blah, Blah, Blah, Drone...