But in short, a session in QlikView is cached and stored into memory until QlikView reaches some limits after what QlikView starts to purge memory. In terms of browsing session, there is a setting in the QMC, System, Setup, QlikView Servers, Documents, named "Session Recovery" that allows the user to keep the same state of the document after closing the browser.
That is done by creating and storing a bookmark in the server so it is applied automatically the next time the user logs in to that document, and sees the same where he left.
The working set limits are limits that the OS use to control processes in Windows operating systems. The working set low means that until that amount of RAM, QlikView will be free to use as many as he needs (by default 70% in a dedicated computer). The high limit means that from that limit, QlikView will start to purge some old, less used results in order to load newer ones. But take into account that these are limits for the QlikView Server process, and that there will be many other operating system processes running at the same time, as well as the rest of QlikView services (Distribution, Web Server, etc) and all of them using more or less RAM, so leaving as they are is a good idea.
Of course, timeouts can help you control how much time a document should reside in memory before being unloaded, and how much time should a user session be "alive" regardless of activity, (which by the way may affect the use of Session CALs, for example...).