Multicast IP-traffic is a form of traffic in which 1 (or multiple) IP-address send ip-packets to several recipients at the same time. This traffic is done via a sort of 'broadcast' so the recipients do not necessarily be known by the sender. If you for example send a mail from your VPS to 100 different addresses this is not multicast traffic - you know who the recipients are and (via DNS) what the IP-addresses are. You can view multicast IP as a TV channel: it's being send out to a complete network of available TV's, but the broadcasting station has no idea whose watching and where these viewers are located.
Multicast traffic is send to IP's that are located within the 224.0.00/4 range (or ff00::/8 for IPv6). On Wikipedia you can find a complete list of often used multicast IP addresses and their purposes.
Please note! The use of multicast IP-traffic is not allowed on our network. Because this sort of traffic has a noticeable impact on (the capacity of) our network infrastructure, we block this sort of traffic in our firewall by default. Multicast is only used for very specific purposes and in 99,99% of all possible applications on your VPS, you will experience no impact from our block on multicast IP-traffic.