A multicast is a type of transmission in which a single traffic flow is sent to multiple recipients – in other words, a one-to-many technique. In the world of TCP/IP, multicast transmissions are a Network layer concept, using the Internet Group Management Protocol (IGMP) to manage which systems will ultimately receive a multicast, and which routers will forward it.
You may also recall from other articles that switches will, by default, forward all broadcast, multicast, and unknown destination frames to all connected ports. While this may not seem unreasonable at first glance, imagine a multicast being forwarded to literally hundreds of ports when only one or two hosts actually need the data being sent. Obviously this is wasteful, and some technique is required to both reduce the amount of work required by individual switches, and the number of systems that need to process unnecessary traffic.