What does Leave and End Session mean?

Only a Host can End a session, but any user can Leave.

Leave Session

Clicking Leave Session will allow any other user still in the session to continue after you leave.

End Session

Only the session host can end the session. 

Clicking End Session will immediately end the session for all users. Anyone still in the session will see a message that the session was ended by the host and sent back to the GMS dashboard.