IP multicast is a method by which data can be broadcasted to a select group of hosts using a single transmission. These days, it's primarily used for IPTV and satellite connections. In theory, it could be used for any kind of streaming broadcast-type media (e.g. internet TV, internet radio, teleconferencing, and other Video-over-IP applications), and the more hosts on a network that subscribe to a multicast stream, the more efficient it becomes.
However, the only consumer-level implementation of it that I know of is IPTV. In this case, the user's set-top box is the multicast/IGMP client, and their home router would be the multicast/IGMP router. If you only have a single set-top box, I don't think you need it enabled, but if you have more than one, then it allows the upstream server to send only one transmission to your router to serve all of your set-top boxes. So if you had 4 set-top boxes at home, and each set-top box was turned onto the same channel, you could reduce the total bandwidth usage by 75%.
The nice thing about multicast vs. broadcast is that the multicast router knows to send out each multicast transmission only to the devices that have subscribed to it. So your LAN won't be flooded with useless broadcast traffic.
There aren't really any risks to leaving it enabled. But for now you're not likely to get any benefit from it unless your home/office is subscribed to IPTV or some other IP multicast service.