A backup of an Internet site is a copy of its content, that is kept on some other hosting server and could be restored if something happens. This function is very useful, considering that you can never know if a script update won't go wrong or if you will not eliminate something by accident - a file, a folder, a database entry, and so forth. If your site is backed up, it could be restored the way it was before the problem appeared, so there will not be any damage, or at least it shall be small, based on the specific situation. Keeping backups on your laptop or computer is not very feasible, since you'd have to do it at least once every day and you can still lose information if your last backup is not recent enough. In this light, you will need to rely on your web hosting provider, so you must double-check their policy on the backups, because some providers create backups just once a week, which will do no good if something wrong happens with a website that is being used and updated all the time, like an online store, for example.