Channels are how messages are sent and received. Clients that subscribe to a channel will receive messages that are published to that channel.
Channels are very lightweight and flexible. Channels exist merely by using them. Just publish or subscribe to a channel name and nothing more. And, use as many channels as your app requires. There is no additional charge for the number of channels you use. But, you should use them in a way that makes the most sense for your use case.
Furthermore, a channel is... just a channel. What you use it for is completely defined by your application requirements. For example, a channel might be used for 1-to-1 private chat, many-to-many group chat, 1-to-many broadcasting, or many-to-1 fan-in use cases. And a channel that is being used for private chat can suddenly be used for group chat. In other words, there are no predefined properties that need to be configured with channels. It is just up to you to allow more users to start subscribing and publishing to that channel.
To put it simply, channels are just communication devices for which you determine the actual usage.