What is an SID?

So if you don't already know there is a small tag at the bottem left of your screen in the lobby something along the lines of

SID: (insert a bunch of numbers and letters) 

What is this used for and can someone do something with my account if they know these digits? Please anwser I'm worried

Thanks 

Noobl3fty26

[Superuser Edit: Updated title for clarity]