Suspend to disk file.
Before Windows 2000, the only way to long-term "suspend" a system was to have this file that essentially accepts the contents of the RAM. When the system is powered back on, it is (theoretically) restored back to the condition it was when you left it. Works well for people moving several times in a day to avoid powering the system on and off several times a day. It's essentially a "sleep" mode that allows it to recover faster than going through the entire startup process.