"Put simply, the light body is our vehicle to graduate from the human condition to return to source. If the goal of each soul is to merge with God, then the activation of our light body is what gets us there." -- https://www.gaia.com/article/what-is-a-light-body" onclick="window.open(this.href);return false;
And in Christianity:
"The eye is the lamp of the body. If your eyes are healthy,[a] your whole body will be full of light." -- Matthew 6:22
[a] The Greek for healthy here implies generous.
There are probably other descriptions of what the light body is in other traditions and teachings, but I will use those two quotes to begin with for my own interpretation (speculations

The ordinary physical human body is made of atoms. The lightbody is made of zero point energy. Here is a short video about what zero point energy is:
[youtube]https://www.youtube.com/watch?v=m1FUraLn4v0[/youtube]
The vacuum catastrophe mentioned in the video is the huge difference between measured energy in empty space and what is predicted by electrodynamics theories. Are the theories wrong? No, not if the huge difference is simply an effect of how the energy is measured. When measuring an average of random fluctuations the total amount of energy will be very small. And when adding the fluctuations as absolute values the sum becomes huge.
Empty space contains enormous amounts of energy and appears clear and without much energy because of the cancelling out of the random fluctuations on a macro scale. In order to tap into the energy in empty space, the zero point energy, the random fluctuations need to be manipulated so that they become less random, and then vast amounts of energy can be extracted.
Atoms are zero point energy shaped into particles. And our physical bodies are to a large extent limited to the limitation of traditional physical particles. Our consciousness is primarily located in the brain which in turn is limited by physical particles.
Out of body experiences, OBEs, indicate that even with our physical bodies consciousness can move outside the body. This is possible because the consciousness is then manifesting directly in the zero point energy of space.
The lightbody is simply consciousness expanded from being limited by physical particles into being manifested directly in the zero point energy within the physical body. And since zero point energy is the quantum soap that the whole universe is made of, the lightbody is directly connected to the rest of the whole universe and to other universes.
Activating the lightbody is about making consciousness "the eye that is the lamp of the body" awaken within the whole physical body.
[youtube]https://www.youtube.com/watch?v=YEuA5Y_Cc88[/youtube]