Has to do with those points up top that allow you to do nifty things.

From what I read today, I think the ability allows you to meditate.  Once you're sitting there for more than 10 seconds the points will start increasing.  Are you asking about the ability or general info?

The roleplaying explaination is:
Jin is the lifeforce of the world, which monks draw upon to create various effects.

The technical explaination is:
Jin is a sort of powerpoints, its like mana but for monks.
Jin builds up as you use your special attacks (the correct amount is listed in the description of the skill), or at later levels using the meditation skill.
The amount of jin you currently have is displayed in the bubble with numbers below it, right next to your hp/endu bars.
Jin can be saved up between battles, but decay over time.
The max amount of jin you can get increase as you level higher.