A backup of an Internet site is a copy of its content, which is kept on a different web server and can be restored if something happens. This option is very practical, given that you can never know if a script update won't crash or if you won't delete something unintentionally - a file, a folder, a database entry, etcetera. When your site is backed up, it could 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 small, based on the specific scenario. Keeping backups on your computer isn't very feasible, because you'll have to do it at least once each day and you can still lose data if your last backup isn't recent enough. In this light, you'll need to rely on your web hosting provider, so you ought to double-check their policy on the backups, due to the fact that some providers generate backups only once a week, that will do no good if something goes wrong with a website that's being used and updated at all times, like an online store, for instance.