Hey i will give a nice example to clarify more -
Say you went at a shopping mall, filled your shopping cart with things,
suddenly you got a urgent call from home and left for home keeping the cart as it is there.The person at the shopping mall wont have a idea and will not even touch your cart. But after some time, may be at the end of the day he will disassemble your cart.
Similary, if you are into a SESSION on a website, and suddenly close the browser, your session is still maintained. The server/container will not touch it for some time - determined by the setMaxAgeInterval( ) [if its set]. After that interval expires OR after some CONTAINER speecific time expires, the container will INVALIDATE the session
Hope that helps you understand even better