thanks
This is the number of seconds for sessions to last after they haven't been accessed. If a session is not accessed for too long, it is said to have "timed out". Anything higher than 2400 is recommended.
So, why would I want anyone to be timed out exactly? How about 100000?
Well, for one keeping unused sessions alive needlessly is bad for security, and secondly you also tie resources in to handling sessions so it's sensible to kill them off at some point. It's all about maintaining a sensible balance.