If you have multiple SAP sessions open from one system and one of them gets hung up on a transaction or program and you would like to stop that one session without closing them all, you can use this transaction to end the one session.
Let's say you want to end session three. You can go to either session one or two and enter this command in the transaction box: /i3
This will immediately close the hung up session (session three in this case).
Or you can do this the long way by going to SM04 and double-click your userid, select a session and hit 'end session'.