Electricity sparks when it travels through the medium of air and gets manifest. When it is contained in the wire, it is not seen to the naked eye. More subtle than electricity is the self. Atma Bodha by Sri Adi Sankaracharya says that this light of consciousness is alone by itself and it is spread everywhere, within the body and without.
Even though this thought is very simple, the Upanishads and other texts keep reiterating this point again and again as constant remembrance is the only way we can abide in the self. As Sri Ramana Maharshi says in his composition called Saddarshanam: remembering it is the way to abide in it.
This light of consciousness is not visible to the naked eye. That is the reason why generally people are unaware of its existence. In fact, this is what Vedanta means by ignorance. As a manufacturing defect, humans are unaware of the self, the presence of life and the light of consciousness within. Then how is it known? It is through the scriptures or books of divine knowledge and the Gurus who interpret them to bring that knowledge down to our understanding that this is revealed. It is not otherwise known by the senses or by conjecture.
How does this consciousness express itself? The Atma Bodha says, just as a burning hot ball of iron which is black in colour, but shines like molten gold, the fire which has no shape appears round like the ball, the light of consciousness which has no form takes the shape of the body and the body which has no life.
What is the effect of this meditation on consciousness as shining within and without. The Acharya concludes that whatever we meditate on, that we become. Though it may be true for material objects and qualities of the mind only to some extent, meditating on the Brahman turns the mind completely as the substance of light and consciousness, it really is.
The essence of the whole verse in brief calls us to meditate on the Brahman which is alone by itself, spreading all around and within, shedding its light on this whole universe of moving forms, names and ideas. This Brahman alone shines on all our thoughts and experiences within taking the form of the body, mind and intellect just as the fire takes the shape of the iron ball and lends its light and heat to that cold and dark metal.
Constant meditation on this truth takes the mind higher and higher, dropping all the mundane names and forms that it always mulls over to the presence of that one consciousness in every object, person, thought and experience.