Hmmm. Long explanation.
In short, certain parts of an application are visible to users – this is the front-end. Other parts operate behind the scenes and are not visible to the user. This is the ‘back end’ of the app.
As an example, you see the Instagram app where you can interact with it in many ways, like uploading an image. But once that image is submitted, a whole lot of processes kick in that are invisible to you.
Hope this explains it.