Backend
The part of a system that runs on the server — what the user doesn't see directly.
When you load Instagram, part of the app runs on your phone (frontend) and part runs in Instagram's data centre (backend). Backend handles the database, authentication, business logic, third-party integrations.
Typical backend languages: PHP, Python, Node.js, Go, Java, Ruby, Rust. Differences in pace, ecosystem and performance — but functionally equivalent for most small projects.