A server is a computer that stays available to answer when other computers ask for something: show this page, save this form, look up this account. It can be a big machine in a closet, a rented box in the cloud, or your own laptop serving local work. In web work it usually runs the backend half of the frontend and backend split, answering whenever your browser requests a page or calls an API.
The front desk in a building that’s always staffed is the everyday version. People walk up and ask for things, and the desk is the always-available place where every request gets handled, whoever happens to own the building.
Why you care
“It works on my machine” is a checkpoint; the server is the finish line. Your laptop may have the right files and setup while the live server is missing a secret (a stored password or key it needs), running the wrong runtime, or pointing at the wrong database, so the live system can fail even though the local version looked fine. The more important the work, the more you care whether that computer is stable, secure, monitored, and easy to roll back.