Backend is the place where most work and sweat takes place while creating something for the web. This work and sweat translates into first planning and then a lot of coding. The backend code runs on the server, not in your browser. Backend functions are ususally triggered by request sent by the browser. A response is generated by the code on the server and sent back over to you.
Take for example a shortest route calculator website running in your browser. You want to know how to get from Carlisle to your web designer in Edinburgh. Your browser will not know. So the words Carlisle and Edinburgh are sent to the server where the program running on the backend actually does all the calculations. Then the result is sent back to your browser and displayed on a map for you.