Hello Mark.
The lesson here is simulating a situation where there is a unidirectional link such as is often the case when using satellite connections. All traffic on the satellite link goes from the internet towards the user, whereas all upload data goes via an alternate physical link. So, in order to simulate this situation, all incoming data on R2 G0/2 interface is blocked.
Having said that, let’s get to your questions:
What the R2(config-if)#ip igmp helper-address udl GigabitEthernet 0/2 command is doing is providing a helper address (similar to a DHCP helper address) that according to Cisco
“helpers [yes helpers] the report to the IGMP querier associated with the UDL interface identified in the ip igmp helper-address interface configuration command. This command should be used on every downstream router on every interface to a potential multicast receiver.”
You can find more info about this at this Cisco Documentation link.
So it is used as a destination address for responses from R1 to multicast queries from H1 that reached R1 via the back-channel. If this was not configured, then any IGMP packets destined for H1 would reach R2 and would be dropped as these did not originate from the Gi0/2 interface. Rene’s explanation of the command may be a little bit confusing, I will let him know to revise.
You do not need to specify whether the direction is sending or receiving. IGMP will learn which direction it is based on the nature of the connection. In other words, the direction is determined based on which direction the packets are actually flowing.
I hope this has been helpful!
Laz