I recommend you to read this:
Client here refers to a multiplayer game context. In a networked game (multiplayer game) each player is a client, each player has an instance of the game running and they connect to a server. The server tells every client the information they need to keep the game running, e.g. the position of every other client (player) in the map, their health, score, etc.
I also recommend you to watch this youtube playlist from the Unreal Engine channel. At least the first video
here’s a picture from that video that shows the client server relationship
