What is Backend
The half of an application users never see: server code that stores data, enforces the rules, and answers requests from the screen.
In plain terms
Backend code runs on servers the company controls. It handles the parts where mistakes are expensive: sign-in, payments, data storage, and serving many users at once. Common backend languages are Python, Go, Java, and TypeScript, paired with a framework and a database such as Postgres.
What it tells you about a developer
Strong backend developers think about correctness, failure, and scale: what happens when a payment retries, or when traffic triples. The skill transfers well across companies and across languages, because the hard part is systems thinking. Backend depth usually anchors a senior generalist, and it is the most common dominant side under a full-stack title.