Hello Kenneth
Under the BGP configuration, when you configure an address family, you are choosing to configure the BGP session for a particular type of traffic. If you don’t use the address-family command, and you configure BGP directly, then you are actually configuring the IPv4 address-family, since this is the default.
By issuing the address-family ipv6 multicast command, you are placing the router in IPv6 address family configuration mode. Specifically, you are indicating to the router that you want to create IPv6 multicast routing sessions.
This particular command is used when you want BGP to advertise IPv6 multicast address prefixes.
For more information, take a look at this Cisco command reference that further explains the use of this command:
I hope this has been helpful!
Laz