A backup of a website is a copy of its content, that is kept on a different web server and can be restored if something happens. This function is very effective, considering that you can never know if a script update won't crash or if you will not eliminate something accidentally - a file, a folder, a database entry, and so on. If your site is backed up, it could be restored the way it was prior to when the issue appeared, so there will not be any damage, or at least it will be little, depending on the particular scenario. Keeping backups on your personal computer isn't very feasible, since you'd have to do it at least once a day and you can still lose information if your last backup isn't recent enough. In this light, you will have to rely on your web hosting company, so you need to double-check their policy on the backups, given that some companies generate backups just once every week, which will do no good if something wrong happens with an Internet site which is being used and updated at all times, like an online store, for example.