The heal ability is automatically used and targetted at a near-by unit with low-health. The Meric will attempt to heal the unit with the lowest health first. If needed, a Meric will move towards a wounded friendly unit unless told to hold. This ability is free to use and is on by default. It heals approximately half a bar of health per heal.
Due to a glitch in the game, if multiple Merics attempt to heal all at the same time, it will act as if only one Meric actually healed. Thus, to achieve the fastest healing rates, the Merics should be moved so that they all do their heal animations at different times.