Nice introduction, well explained.
Just a note, for the other readers, who don’t know what is hidden behind this back-end stuff.
So... Back-end programming is when you write code for the servers, aka the "instructions" to send to the browser on what to display.
This is only the beginning of the story. I guess we all saw the iceberg metaphor before.
Then replying to a browser (or a mobile app) is only the first layer beneath the surface. And when you dive deeper, backend also covers when this first layer needs to delegate more complex tasks (like long-running stuff, asynchronous stuff, lots to stuff) to other systems.