Zen is the Archer class equivalent of Rage and Charge.
The left arrow shows the Zen charge bar and the right arrow shows the hourglass for skipping turns.
Zen must be charged up by skipping a turn (Click on the little hourglass in the upper right corner).
Charging Zen will increase your accuracy and allow for higher damage hits.
When an enemy damages you, your Zen will drop. This also applies if you take damage from a trap triggered in a dungeon. However, this only applies if hit points are lost. If an enemy tries to attack you, but deals no damage, your Zen will not decrease.