A Multipoint Control Unit (MCU) is also referred to as a Bridge or Virtual Meeting Room (VMR). Think of a bridge as the hub that connects multiple participants (spokes) in a conference.
You would use a bridge whenever you have more than two sites in total (otherwise they would usually just call each other directly).
Bridges can connect many VC participants using hardware units, software clients, web browsers, and mobile devices, as well as phone connections, and show up to around 9 sites at the same time.