IGMP, or Internet Group Management Protocol, is a communication protocol used by IP networks to manage the membership of hosts in multicast groups. It allows devices to join or leave multicast groups, enabling efficient data transmission to multiple recipients without sending multiple copies of the same data. When a device wants to receive multicast traffic, it sends an IGMP message to indicate its interest in a specific group. This helps routers manage the flow of multicast data, ensuring that only devices that want to receive the data are included, thus optimizing network resources and reducing unnecessary traffic.