What does having a network that would allow a multicast address to find host computers mean?

I'm dealing with a scenario with a service that's saying the following:

"... [this service] uses multicast address 239.170.165.207 on port 49838 to find host computers, so your network must allow this."

First, what does this actually mean? It appears that 239.* is a reserved address for multicasting, but I'm not groking what this translates into for application on our network office since we're hitting walls wherein the service is not being recognized.

To be specific, we're attempting to develop some software with Google Daydream Instant Preview as per this. When using the WiFi mode, and while

"running the Unity editor with Instant Preview, it should be discoverable by an Instant Preview app running on a phone connected to on the same WiFi network. Tap the drop down field under “Preview via…” to see available computers to stream from.

"Tap on a computer name to connect to it. Instant Preview should operate the same way as if you were connected via USB."

This doesn't work and I'm not seeing where the problem may be. We're running a Linux server that acts as a firewall and gateway with some some switches and a wireless router that bridges the traffic over (acting as a wireless switch effectively). Nothing should really be doing anything peculiar, however I've not really dealt with multicasting situations before on our network and thus I suspect I'm missing some detail here.

Thus I'm hoping I can understand what is supposed to be going on here to perhaps get to the bottom of what may be preventing this from occurring properly on our network.