Could you give me some context? Are you reading that somewhere?
In general, offsight refers to when you store something in a different physical location then the original medium. AKA cloud storage. Onsite is when you store it in the same location, like a CD or flash drive connected to the computer.
Its good to incorporate both types into your backup solution. Large files are better backup onsite, so your not using up bandwidth you don't have to, and recovery is easier. But then you want to keep some stuff in the cloud so it can sync between devices and give you access to backup files from remote locations (hotel, work, home, etc). Hope that answers your question. 