Join or subscribe in groups for members, ???

In the groups, Log in -members can choose  to  join and subscribe

Is not clear for me what's the difference between those

join the group is clear....
But subscribe to what? ( they are already a member)


Tom 

I also want to know!

Subscribe to a group (or any other content) means that subscriber will get news about the subject, such as if content was edited or comment was posted.