Channels can be as broad as you like such as sales and marketing or more of touchpoint basis like Facebook, Twitter, LinkedIn.
Once the channels are created the options will be available as a dropdown in tasks, the idea being users will plan content creation against these channels.
NOTE: This does not push content to that channel it is just for planning purposes.
To create your channels, go to the planning section and select 'Channels' and new to add a new channel.
Be sure to add an icon and select the colour to make them standout!