Internet Group Management Protocol (IGMP) is a communications protocol used by hosts and routers. With IGMP, specific network devices are identified to receive network traffic, as opposed to network traffic being sent to every device connected to the network.
IGMP is required for this product because the total amount of network traffic across the network needs to be reduced to avoid impacting the other connected devices. Without IGMP, the video extender would send the video signal to each of the devices on your network.