This seems to be a subtitle from 'Wall-E'. It is information for deaf viewers and is a short way to say "A song called 'Put on your Sunday clothes' is playing".

It means to wear your clothes.

Hmm, I don't know about the "playing" part, but "to put on your Sunday clothes" means to dress formally, to dress well. It is a reference to how many people go to church on Sundays, and generally people dress in very nice and formal clothing for this occasion.