A backup of a site is a copy of its content, that is kept on a different hosting server and could be restored if something happens. This feature is very handy, because you can't know if a script update won't crash or if you will not eliminate something accidentally - a file, a folder, a database entry, and so forth. If your site is backed up, it could be restored the way it was prior to when the issue appeared, so there will not be any damage, or at least it'll be small, based on the particular situation. Keeping backups on your PC is not very feasible, as you would have to do it at least once daily and you can still lose information in case your last backup isn't recent enough. In this light, you'll need to rely on your website hosting provider, so you have to double-check their policy on the backups, because some companies create backups only once each week, which will do no good if a problem occurs with a website that's being used and updated all of the time, like an online store, for instance.