Chakra is the resulting form of energy when two other forms of energy are mixed together. The two energies are referred to as "physical energy" and "spiritual energy". Physical energy is collected from the body's cells and can be increased through training, stimulants, and exercise. Spiritual energy is derived from the mind's consciousness (i.e. the soul) and can be increased through studying, meditation, and experience. Having these two energies become more powerful will in-turn make the created chakra more powerful. Therefore, practising a technique repeatedly will build-up experience, building up one's spiritual energy, and thus allowing more chakra to be created. As a result, the ninja is able to do that same technique with more power. This same cycle applies for physical energy, except this time, instead of doing things like meditation, the ninja could, say, do push-ups.
The general term for the source of chakra, the overall physical and spiritual energy an individual has, is called "total stamina". The maximum amount of chakra an individual can create and retain at a given time is called their "chakra level".