CHI is a substance in Legends of Chima that gives animals special powers. These special powers make them feel so good that they feel the need to fight over it. Some of them even use CHI to power their vehicles.
As a drug Edit
CHI is known across the world of LEGO as an easy, naturally-produced drug, much like marijuana or steroids. Many people use it. A common example of this is the Gorilla Tribe in Chima, who appears to always be stoned.
CHI can also be used as an explanation to any uncertain or not able-to-be explained causes in LEGO, in which it is always safe to assume either the creator or the characters are on CHI.