Channel tanking is when a Monk sacrifices positioning in order to maximize energy gain from Channeling. The term was popularized in Heroes' Ascent when that skill was the preferred form of energy management, since Monks could take advantage of balling-up of foes caused by the terrain of particular maps.
This is generally a very effective form of energy management, although it does increase the danger the Monk may face. In HA, however, most teams are mixed well in with their foes, so that the distinction between front and backline refers only the role a person plays in their party rather than actual position on the field.
The term is somewhat vaguely-named, having led many inexperienced players to believe that the player is literally to "tank" an enemy in order to maintain their energy gain. Only in very rare cases (or when using a skill appropriate to the threat, such as Guardian against a Warrior) is this advisable. Kiting is still better than standing and taking the damage.