The backend is the server side of a web application.
What is the backend?
That said, some languages are more popular and practical than others. If you're running your own server, you have a ton of flexibility but plenty of headaches. If you're deploying to the cloud (which we will be doing later), you may be restricted to those languages which your cloud provider has installed on their platform... it doesn't do you much good if the servers you're "borrowing" from can't understand your code!
What is the difference between front-end and back-end development ?
Why are we required to install back-end languages to our computer (besides to run them)?