Channeling is the act of receiving transmission and information from non-physical beings. When one channels they are plugging into a higher frequency, sometimes referred to as the Astral, Causal, Akashic, or higher-vibration planes or dimensions.
It's like tuning in a radio. Each station, or energy plug-in, brings a specific frequency of information that we sometimes associate with colors, chakras and tones.
A few famous channelers are Jane Roberts (of the Seth books), and Edgar Cayce.
Anyone can learn to channel but this gift should be approached with thoughtful consideration; not in the spirit of a parlor trick. In the light of infusing and inviting a non-physical being to exist in the same time-space with you, using your entry points to communicate truths and wisdom, you should always seek to commune with only those entities that resonate with your higher good. Accept nothing other than the pure and best for your experiences with this layered reality. And remember, you are in control and you can choose with whom, how, and when you channel!