Front end and back end describe program interfaces relative to the user.
The front end, here, is the appearance of your website. It is the graphic design and HTML portion—some people call this the user interface or UI.
In contrast, the portion of the application you or your developers work with is the back end. The back end handles the dynamic parts of the site, such as a newsletter, an administration page, a registration database, a contact page, or more complicated web applications. Your back end interfaces with your UI and makes your website work.