From Vanguard Wiki
Jin is a special pool used by Monks and Disciples as the cost to perform certain special abilities and attacks. Jin pools are built through the use of certain skills during combat, and decrease when out of combat. Not all abilities increase Jin however.
Much of Jin itself is heavily involved in the culture of Kojan and parts of Qalia. The spirits of Lao'Jin and Ra'Jin are at the core of this phenomena.