A chakra is an energy centre.

There are 7 main chakras, each one is associated with a color and a vibration interms of physical, mental , emotional and spiritual bodies, they are what feed your aura.

You can simply put this is as culmination of different vibratory energies located at 7 different main points in a human energy field, they in turn feed the physical functions of associated organs for each chakra.

The quality of your chakra determines your quality as a person. In essensce they have to balanced. an overactive chakra and an underactive chakra is what causes problems at the physical level.

any disease forms first at the energy level, it takes sometime to manifest on the physical, that is why energy healers can detect diseases even before they occur.

this subject is vast and cannot be explained in one post. you might want to do a google search on this subject to know more