A backup of an Internet 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 practical, considering that you cannot know if a script update won't fail or if you won't erase something accidentally - a file, a folder, a database entry, and so forth. If your site is backed up, it can be restored the way it was until the issue appeared, so there will not be any damage, or at least it shall be small, based on the particular situation. Keeping backups on your PC isn't very feasible, as you'd have to do it at least once each day and you could still lose information in case your last backup isn't recent enough. In this light, you'll have to rely on your web hosting provider, so you have to double-check their policy on the backups, as 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, such as an online store, for instance.