A backup of an Internet site is a copy of its content, that is kept on a different server and can be restored if something happens. This option is rather effective, considering that you can never know if a script update will not fail or if you won't delete something accidentally - a file, a folder, a database entry, and so on. When your site is backed up, it may be restored the way it was until the problem appeared, so there will not be any damage, or at least it will be small, dependant upon the particular scenario. Keeping backups on your laptop or computer is not very feasible, as you'll have to do it at least once daily and you can still lose data if your last backup isn't recent enough. In this light, you'll have to rely on your web hosting company, so you have to double-check their policy on the backups, because some companies create backups once a week, that will do no good if something wrong happens with a website that is being used and updated all of the time, like an online store, for example.