A backup of a website is a copy of its content, that's kept on a different server and can be restored if something happens. This feature is rather efficient, because you can't know if a script update won't go wrong or if you won't eliminate something by accident - a file, a folder, a database entry, and so on. If your website is backed up, it could be restored the way it was before the issue appeared, so there shall not be any damage, or at least it'll be minimal, dependant upon the specific scenario. Keeping backups on your personal computer is not very feasible, because you'd have to do it at least once a day and you may still lose information if your last backup isn't recent enough. In this light, you will have to rely on your hosting provider, so you have to double-check their policy on the backups, given that some providers create backups just once a week, which will do no good if something wrong happens with an Internet site that's being used and updated all of the time, such as an online store, for example.