The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light.
--Matthew 6:22, King James Bible
We have heard about illuminating all seeing eye before. But the eye in question for this article and in the Bible quotation above is not conspiratorial rather it's the really illuminating one. The quotation above has been interpreted in various ways. The one I like and find appropriate in particular refers to the third eye, ajna chakra or shiv-netra. When Jesus says that 'if thine eye be single' he means that if 'you have awakened the single eye'.
Jesus says that your body will become full of light--that means when you have awakened your third eye your body will become full of consciousness or you will get your real body which is full of light without the dint of matter or Maya. You will become illuminated and wake up from your slumber of ignorance. 'Full of light' means full of consciousness, full of joy and full of knowledge of self. The location of the third eye is between the two eyebrows and it's considered to be seat of consciousness. Physical counterpart of this eye is Pineal gland in the body. It's considered to be sixth chakra in the subtle body. On a spiritual level it's reflected in the qualities like foresightedness and when activated by help of meditation it confers one knowledge of self . This eye makes one an all seeing one and an enlightened one or an illuminated one, which is where-from the conspiratorial all seeing eye took its nomenclature.
In general when meditating or otherwise the subtle body center at which you focus or the vibration frequencies with which you keep yourself associated, are the one towards which you reinforce your body and mind. For example if you are consistently engaged in thinking about welfare of humanity and with an open heart you love one and all--your heart center gets activated; similarly when you keep on focusing between eyebrows behind the eyes--your third eye gets activated and you start seeing light there. Initially the light might be flickering and hazy but with the practice it becomes crystalline and more and more radiant and it makes your body full of light.
Yogic texts and other references by masters suggest that the one who starts practicing meditation at the eye center, need not activate other chakras. The third eye in itself, when activated, gives advantages of working on all lower chakras and makes you full of light of wisdom and consciousness.
image source: here