NIS - signed in vs not signed in?

I have NIS and a very basic question: in the right hand upper corner I can "sign in" and after signing in, my user name is displayed. I can also sign out and the program seems to be running still, just text "sign in" displaying again. What's the difference between using NIS "signed in" vs "not signed in"? I haven't been able to find any clear info on this.