When your system runs low on RAM because an application like Firefox is taking too much memory, Windows moves the least used "pages" of memory out to a hidden file named pagefile.sys in the root of one of your drives to free up more RAM for the applications you are actually using. What this actually means to you is that if you've had an application minimized for a while, and you are heavily using other applications, Windows is going to move some of the memory from the minimized application to the pagefile since it's not being accessed recently. This can often cause restoring that application to take a little longer, and your hard drive may grind for a bit.