Could you be a bit more precise in what you need to know as this question is way to general. Also this is a kind of 1.0.1 concept in Computer Science.
If you don’ t know anything about the Client-Server model I advise you to start here: Client–server model - Wikipedia
Then as LostScout tells you below, learn about UE4 networking basics and multiplayer setup.
As a brief answer, game wise, the server application is responsible for hosting multiple client sessions and run the world logic by its own. It has no renderer as it is not supposed to show anything. The server is highly CPU oriented as it as to handle multiple player actors and sync them together.
On the other side clients are applications that connect to the server and replicates and render the world simulated on the server. The UI for instance only runs on clients as it is a pure client concept.