There are generally two ways people use the term: client side modifications that assist or automate what a human would do, or direct communication with the server. There can be overlooked things that allow you to talk to the server in a way the official client never does, but can be used to get significant advantage. Since the server is usually what hold the "truth" of the game world, just modifying the client won't in and of itself give you ability to alter the game state in beneficial ways.