A backup is a copy of a site that is kept on a separate hosting server and could be restored if a problem occurs with the live Internet site - a failed script update, an unintentional deletion of a file or of the whole database, etcetera. Restoring the site the way it was will remove or decrease the damage the problem may have caused, that is by all means a much better option than having to reconstruct your whole site from scratch. While you could download a copy of your content on your computer, keeping a backup is a feature that nearly all hosting companies include as part of their plans. It's important to check how frequently they do that, though, because some providers generate a backup only once every 7 days, which can be far from enough for a booking Internet site or an e-commerce portal in which the information is updated every day. You should also see how fast and easy a backup may be restored, which could be critical if some issue appears on your site.