What is a Git Revision Expression?

So, I am using Git GUI to make a repository. But I cant find ANY trace on Google, the Documentation, or anywhere else what a 'Revision Expression' is, and it is required to create a new Branch.

Also, it seems that this is used many other places in the program, so I belive it is important to know.

I did find a question on this on StackOverflow, but the guy never got an answer.