A backup of a website is a copy of its content, that's kept on a different hosting server and can be restored if something happens. This option is rather efficient, since you can never know if a script update will not fail or if you won't eliminate something unintentionally - a file, a folder, a database entry, etcetera. If your site is backed up, it may be restored the way it was before the problem appeared, so there shall not be any damage, or at least it'll be minimal, based on the particular situation. Keeping backups on your computer system is not very feasible, due to the fact that you would need to do it at least once per day and you can still lose info if your last backup isn't recent enough. In this light, you'll need to rely on your hosting company, so you must double-check their policy on the backups, due to the fact that some providers generate backups once each week, which will do no good if something goes wrong with an Internet site that's being used and updated constantly, such as an online store, for example.