Frontend comes from the English word front, meaning the foreground in this context. It really is the foreground, of either a web or a traditional application.
It is always the content shown to the users of a website. In a web application or content management system, this means the displayed articles, modules and other things that the user can read, download and work with. In an online shop it likewise means modules, but above all the goods and products displayed in the shop, the shopping cart and more.
An ordinary user cannot change the website's content from the frontend in any way. Only administrators (owners, managers) of the website can, in the so-called backend, which has an article in our glossary.