A backup of a website is a copy of its content, that's kept on another hosting server and could be restored if something happens. This function is quite useful, considering that you can never know if a script update will not fail or if you will not erase something by accident - a file, a folder, a database entry, and so forth. If your website is backed up, it could be restored the way it was prior to when the problem appeared, so there won't be any damage, or at least it shall be very little, depending upon the specific case. Keeping backups on your personal computer isn't very feasible, due to the fact that you'll have to do it at least once a day and you could still lose information in case your last backup isn't recent enough. In this light, you will need to rely on your website hosting provider, so you ought to double-check their policy on the backups, due to the fact that some companies create backups just once a week, that 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 example.