Ki

Characters with one or more levels of monk will possess a yellow bar under their HP meter, called the Ki Meter. It displays three numbers.

The first number is the current Ki. The second number is the maximum Ki the monk can achieve. Your base maximum Ki is equal to ten times the character's monk level plus forty. To that number, you add five times your Wisdom modifier. The third one is written between parentheses and represents the rate at which a monk's Ki regenerates or decays. A rate of (+1) means you're gaining 1 Ki per six seconds, however, a rate of (-2) means you're losing 2 Ki at every six seconds.

The rate at how much you gain or lose Ki is determined by your Concentration skill. Concentration is a critical skill for the monk. If your Ki is equal to or less than your Concentration skill, it does not decay. For each multiple of your Concentration skill that it exceeds it by, you receive -1 Ki for every 6 seconds. If your Concentration score is zero or negative, it's treated as if it were 1, which makes retaining Ki difficult.

Gaining Ki
There are different ways to gain Ki. The first is by attacking a monster. In general a melee attack that does more than zero damage will generate 1 Ki and scoring a critical hit will net you 3 Ki. If you are in fire stance you gain 2 Ki instead. Way of the Tenacious Badger allows your monk to gain more Ki while damaging an opponent for as long as the monk is below a certain percentage of health and Way of the Elegant Crane increases the Ki a monk will receive when scoring a critical hit. Also, Mountain stance grants Ki on being hit. Ranged attacks will not generate Ki.

The second way to gain Ki is to rest at a Rest Shrine. Your Ki then will reach the maximum sustainable amount which is equal to your concentration score. Zoning in or out of a dungeon will have the same effect, just as your buffs reset so does your Ki.

The third way to gain Ki would be to use the Meditation feat which is gained at level two. This will regenerate your Ki based of the monk's wisdom modifier and monk level. While meditating you can go over your centered Ki up to your maximum Ki.