A backup of a website is a copy of its content, that's kept on a different hosting server and may be restored if something happens. This function is very effective, considering that you cannot know if a script update will not fail or if you will not eliminate something by accident - a file, a folder, a database entry, etc. If your Internet site is backed up, it can be restored the way it was prior to when the issue appeared, so there won't be any damage, or at least it will be small, based on the particular case. Keeping backups on your personal computer is not very feasible, as you'll have to do it at least once daily and you may still lose information if your last backup isn't recent enough. In this light, you will need to rely on your hosting provider, so you must double-check their policy on the backups, as some providers create backups just once per week, that will do no good if a problem occurs with a website which is being used and updated all of the time, like an online store, for example.