This is a backup procedure that should be followed by all sysadmins. The idea behind it is that even if your backup is done but your office burns down the next day, you are still screwed (but not in a good way).
An offsite backup is taking that information away from the location. So if it's in the form of a physical media you'd take that media (be it tape or cd) home with you at night.
If your backups are in the form of a zip or tarball you would store this on another server that is not residing in the office.