Channel tanking is when a Monk moves into the front line to gain increased benefit from the spell Channeling. Most monks use Channeling as energy management in Heroes' Ascent. The more foes around the monk, the more energy they will get back from Channeling.
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 line between front and backline is refers only the role a person plays in their party and not their actual position on the field.