The term backend is used in a great many applications and programs, and it always means the same thing. It is the administration interface, for example of an online shop, used to manage the website from behind the scenes.
The backend is hidden from the ordinary user, who has no access to it. In a content management system, for example, the backend is used to manage the articles displayed to ordinary visitors. In an online shop, the backend is used to publish, add and manage the products displayed on the website.
It is thus a kind of second half of a website, which is used to create everything that is displayed on the pages and seen by ordinary users.
The opposite of the backend is the so-called frontend. You can also find out what that term means in the Glossary under the entry Frontend.