A backup of a site is a copy of its content, that's kept on another web server and may be restored if something happens. This option is very effective, since you can't know if a script update shall not crash or if you will not eliminate something by accident - a file, a folder, a database entry, etcetera. If your website is backed up, it may be restored the way it was until the problem appeared, so there shall not be any damage, or at least it shall be little, dependant upon the particular case. Keeping backups on your laptop or computer is not very feasible, simply because you would need to do it at least once a day and you can still lose data in case your last backup isn't recent enough. In this light, you will need to rely on your web hosting company, so you must double-check their policy on the backups, given that some providers generate backups just once every week, that 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.