A backup is a copy of your information kept somewhere other than the place it normally lives. If a system fails, a file is deleted by accident or a device is lost, the copy lets you put things back as they were at a chosen moment in time.
Two questions matter more than people expect: how recent is the latest copy, and has anyone ever tested restoring from it? A backup that has never been restored is an assumption, not a safeguard. Sensible practice is to keep copies in more than one place, protect them with encryption, and rehearse a recovery from time to time.