Channel Groups is a feature that is part of the Stream Controller add-on. Channel groups is a way to subscribe to multiple channels with a single connection (multiplexing). You can still use multiplexing but you may know that it is limited to about 100 channels per connection.
With channel groups, you can add up to 2000 channels to a single channel group and you can subscribe to a total of 10 channel groups for a grand total of 20,000 channels.
The list of channels in a channel group is maintained on the server instead of the client and channels can be added or removed by the client or the server on all of the clients' behalf (all clients that have subscribed to the channel group).