A backup of an Internet site is a copy of its content, that's kept on some other hosting server and can be restored if something happens. This feature is quite effective, given that you can't know if a script update will not go wrong or if you won't delete something by accident - a file, a folder, a database entry, and so on. When your Internet site is backed up, it may be restored the way it was until the issue appeared, so there shall not be any damage, or at least it will be minimal, depending upon the specific scenario. Keeping backups on your personal computer isn't very feasible, since you would need to do it at least once each day and you could still lose information in case your last backup isn't recent enough. In this light, you'll have to rely on your website hosting company, so you must double-check their policy on the backups, since some companies create backups just once every week, that will do no good if something goes wrong with a site that is being used and updated all the time, such as an online store, for instance.