A backup of a website is a copy of its content, which is kept on some other hosting server and can be restored if something happens. This feature is quite efficient, considering that you can't know if a script update won't fail or if you will not erase something accidentally - a file, a folder, a database entry, and so forth. When your site is backed up, it can be restored the way it was before the problem appeared, so there won't be any damage, or at least it'll be little, depending on the specific situation. Keeping backups on your PC isn't very feasible, because you would have to do it at least once each day and you could still lose data if your last backup is not recent enough. In this light, you'll have to rely on your hosting company, so you need to double-check their policy on the backups, because some providers generate backups just once each week, which will do no good if something wrong happens with an Internet site that is being used and updated at all times, like an online store, for example.