Chakra, or sometimes chi, is a special supply of energy within aliens that fuels their abilities.
The greatest difference between humans and aliens is considered to be that aliens have chakra and humans don't. The chakra within an alien is required for them to perform their abilities needed in battle and even daily life. Each alien has a certain amount of chakra that is increased from physical training that decreases each time an alien uses any of its abilities. The naturally enhanced strength and speed of an alien compared to that of a human seems to be powered by chakra as well, as they decrease drastically whenever an alien's chakra supply is low.
Chakra has five different types, one for each of the five basic elements: fire, wind, lightning, earth and water. A normal alien has only one of these elements that make up their entire chakra supply, and which element depends on their bloodline's abilities. Every ability requires chakra of one specific element to be paid for the ability to be performed successfully.
Each of the five elements have a natural counter, as well: fire is fanned by wind, wind dulls lightning, lightning pierces earth, earth soaks up water, and water extinguishes fire. This was made sure of by the Sage so that one Path would never grow too strong, as they would eventually face their weakness. Therefore, if one alien faces an alien that uses abilities based on the element that is their weakness, that alien will probably lose.
As there are six different paths and only five different elements, the only bloodline that does not have any elements within its chakra is the Deviluke bloodline, as their Materialize ability uses the chakra itself and does not require it to be of any element.