A backup of a site is a copy of its content, that is kept on another hosting server and can be restored if something happens. This option is very useful, given that you can't know if a script update will not fail or if you won't eliminate something by mistake - a file, a folder, a database entry, and so on. If your Internet site is backed up, it could be restored the way it was before the issue appeared, so there will not be any damage, or at least it'll be little, based on the particular case. Keeping backups on your PC is not very feasible, as you'd have to do it at least once daily and you can still lose information in case your last backup is not recent enough. In this light, you'll have to rely on your website hosting company, so you ought to double-check their policy on the backups, because some providers generate backups just once per week, that will do no good if something wrong happens with a site that's being used and updated all the time, such as an online store, for instance.