Hiberfil.sys is a Windows system file used by the hibernation feature. Whenever you hibernate your computer it’s memory contents are written to this file allowing you to quickly resume your computer.
Typically on a server this feature isn’t useful. Although it’s not enabled by default the hiberfil.sys file exists by default and uses several GB of disk space (depending on how much RAM you have installed). As it’s a system file it’s protected and you can’t just delete it. Instead you have to use the following command to remove it:
powercfg -h off