If you ever ran chkdisk in DOS you'll know about lost files. This directory is created at the root of every file system you create when you issue the mkfs command. In UNIX you mount drives and other devices on directories in the file system. Information on what is going on inside the kernel. Anyone can use this disk space but files written here aren't guaranteed to be permanent.
Many systems periodically purge the tmp directory.
Basically this is temporary storage space for programs. I encourage you to look around. See what is here. Don't worry, only the root user can make changes to many of these directories. You'll probably also find directories that you can't access since you don't have the correct permissions.