A client is the player-side software that connects to a game server, renders the world, and handles local input, graphics, and much of the user experience. In multiplayer security, the client is important because it is the part attackers can inspect and modify. Cheat tools often hook into the client, read its memory, or alter its behavior to gain unfair information or automate actions.
From a defense perspective, the client is a weak trust boundary. Anything sent to it may be visible to a determined attacker, so secure game design tries to avoid exposing sensitive state such as hidden player positions. Features like server-side culling reduce what the client receives, making wallhacks harder to build. Even when cheats cannot be fully prevented, limiting client-visible data raises the effort required to abuse the game.