Multicast allows a network to deliver packets to members of a particular group rather than sending a separate copy from the source to every receiver. It requires support and configuration within the participating network. It is different from ordinary internet unicast playback and is not a feature that a player can universally enable.
A practical example
A managed television network can distribute a live channel to receivers that join its multicast group. The network controls how traffic reaches those receivers, rather than treating each viewer as a completely separate download from the original source.
A common misunderstanding
A router checkbox does not make a public internet playlist multicast. The source, network path and receiver must use a compatible delivery arrangement for group-based delivery to apply.
Related terms
Stable technical definition. Editorial approach