A backup of an Internet site is a copy of its content, that's kept on some other server and may be restored if something happens. This function is very practical, since you can never know if a script update won't go wrong or if you won't delete something accidentally - a file, a folder, a database entry, and so forth. When your website is backed up, it may be restored the way it was until the issue appeared, so there shall not be any damage, or at least it shall be little, dependant upon the specific scenario. Keeping backups on your PC isn't very feasible, as you would need to do it at least once a day and you can still lose info if your last backup is not recent enough. In this light, you will need to rely on your hosting company, so you ought to double-check their policy on the backups, because some providers create backups just once each week, which will do no good if something wrong happens with a website that's being used and updated all the time, such as an online store, for example.