Chi is the measure of 'internal energy'. You lose Chi by pushing yourself beyond your normal limits and this is regained over time. Expending large amounts of Chi causes overexertion and risk of death.
Chi is the average of Endurance and Willpower or $(END+WIL)/2$ in full. Chi can be raised or lowered from it's initial amount and certain traits can provide resistance to overexertion or quicker recovery of Chi.