What is *shrug*?
Signalling to another via the medium of text the fact you are carrying out an action, in this case, the movement of raising the shoulders, which highlights the point you are basically saying 'I don't know' or 'whatever' but in a non-verbal manner.
Rob: Guys who is the coolest person ever?
Greg: I KNOW! I KNOW! It has to be rob!
Rob: Yes Greg. Yes it is.