There is a wide variety of internet based media available for people to consume. This can include mainstream sources like TV Station catch-up services, news outlets, and other sources such as video sites like Vimeo or YouTube. In the context of Plex. "Channels" are extensions for the Server that let you watch media from supported services. You install them on the server using Plex Web App, which makes the media available to all Apps on your network.
Some sites may impose geographic-based restrictions, which would mean that you may not be able to view that content if you're located outside of the authorized region. Because of these kinds of issues, Channels that aren't compatible with your Server are not available to be installed.