A backup of an Internet site is a copy of its content, that is kept on some other web server and could be restored if something happens. This feature is rather efficient, given that you cannot know if a script update won't go wrong or if you won't delete something by mistake - a file, a folder, a database entry, and so on. If your website is backed up, it can be restored the way it was until the problem appeared, so there shall not be any damage, or at least it'll be very little, based on the particular scenario. Keeping backups on your computer isn't very feasible, since you would have to do it at least once daily and you can still lose info in case your last backup is not 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, because some providers generate backups only once each week, that will do no good if something goes wrong with a website that's being used and updated all of the time, like an online store, for instance.