A server is basically anything which serves something to something or someone. It could be a person, software or even hardware. Usually when talking about a server in a SD context, we mean a piece of software replying to network requests or a piece of hardware hosting such a piece of software.
What exactly a server is supposed to do is highly dependent on what kind of server it is. If a server is supposed to give pizza to clients in a restaurant, then it's their "work" to fetch pizza from the kitchen and carry it to a table (and not the client's^^)
It's the same for software. Server software has to accept a request and handle it. If the server should serve websites, then it has to use all information to load and build HTML, CSS and JS and send it to the client. If the client requests to delete a user, then the server has to check if the client is authorized to do so.
As for hardware, the server is supposed to be up and running all the time and run an application at all times. It's job is to deliver service uptime and performance.
As you can see from above examples, if you want to know the exact tasks of a server, you will have to be way more specific about what the server should do deliver, so that we can tell you what the server has to handle.