Your normal backups are stored on the server where your website is hosted. Off site backups are stored on a different server in a completely different location. Again this is like an extra insurance policy. For example in 2021 several data centres (where servers are kept) were decimated by fires and all the data was lost. Websites that didn't have extra off site backups were lost and irretrievable and had to be built again from scratch. Pricey! Those sites that had extra off site backs stored on another server were able to reinstate them and get their sites back online.
Once a month off site backups are included in all maintenance plans. These are usually sufficient for sites that aren't updated regularly. If you update your site frequently, or for example have an ecommrece site where it's crucial that all orders and data are up to date, then daily off site backups are recommended.
Let's use a real world example. Let's imagine the worst (this is like insurance after all!). Something has happened to your site and the backups are irretrievable so we need to restore an off site backup. The included off site backups are taken once a month so perhaps the backup was taken on the 3rd of the month (they are all taken on different dates, depending on when your site went live). You need to restore your backup on the 30th of the month. Any changes that were made between the 3rd and 30th will be lost, along with any order information (you will still have some details from PayPal/Stripe).
There is also a chance that the once a month backup fails (backups do fail), which means there are no other backups that month to restore.
Please be aware that I'm not trying to be all doom and gloom! Like most insurance, the chances of you needing it are incredibly slim. It's just my job to tell you all the options so that you can make an informed decision 🙂