In most cases, the software you'll be installing on a server will be something that's network connected. That means it'll be software that listens for network connections, accepts them, processes some request, and returns some result back over the same network connection.
For such software to run constantly, it must run as a service. A service is a process that runs in the background constantly, or until some event or user shuts it down. There are literally tens of thousands of examples of software solutions that provide a (network) service, but we'll explore only one: