A backup of an Internet site is a copy of its content, that is kept on some other server and could be restored if something happens. This function is quite effective, because you can never know if a script update shall not go wrong or if you will not erase something by accident - a file, a folder, a database entry, etcetera. If your Internet site is backed up, it can be restored the way it was prior to when the problem appeared, so there won't be any damage, or at least it shall be minimal, dependant upon the particular case. Keeping backups on your computer system isn't very feasible, because you'd have to do it at least once a day and you could still lose data in case your last backup is not recent enough. In this light, you'll have to rely on your hosting provider, so you ought to double-check their policy on the backups, due to the fact that some companies create backups just once each week, that will do no good if something goes wrong with a website that is being used and updated all of the time, like an online store, for example.