Channeling is a practice in which one opens themselves to an entity allowing it to "borrow ones body to speak to others either thru automatic-writing or verbally.
I do not condone such practices as opeing yourself that much to anything leaves you vulnerable to anything that might happen along your path. Just because it says it is a spirit does not mean it is not something darker. Possession is dangerous and can lead to things such as injury and even death.
Honestly I have never understood why would one even consider this option as it simply is unsafe, leaves you open to anything and everything, and just simply put a danger to your wellbeing both physically and metaphysically. Common sense speaks against this especially with so many other ways to communicate with those passed that are far safer to ones own person and others.