A backup of a site is a copy of its content, that's kept on a different hosting server and could be restored if something happens. This feature is rather effective, considering that you can't know if a script update shall not crash or if you won't erase something by accident - a file, a folder, a database entry, etc. When your Internet site is backed up, it can be restored the way it was before the issue appeared, so there won't be any damage, or at least it'll be very little, based on the particular situation. Keeping backups on your laptop or computer isn't very feasible, due to the fact that you'd have to do it at least once daily and you can still lose information if your last backup isn't recent enough. In this light, you will have to rely on your web hosting company, so you ought to double-check their policy on the backups, since some providers generate backups only once every week, that will do no good if something goes wrong with a site which is being used and updated constantly, such as an online store, for example.