Most of the energy available to sustain life comes from the fusion reactions of the sun. Some of the sun's energy is expressed as heat, and some as light. When the energy reaches the Earth, the light is absorbed by green plants and used to fuel the reaction of photosynthesis. Photosynthesis converts carbon dioxide and water into oxygen and sugars. These sugars, together with the starches they form when they combine in large numbers, feed most of the life on this planet.
The light is taken in by tiny organelles inside each cell of a green plant (mostly in the leaves) called chloroplasts. Inside the chloroplasts, pigments like chlorophyll, carotene, and xanthophylls absorb different wavelengths of light depending on their color. Chlorophyll is the most common pigment, and it absorbs all kinds of light except for green, which it reflects. This is why most plants are green. In the autumn, the chlorophyll starts to decay as the leaf nears the end of its life. The other pigments are then visible, turning the leaves orange (carotene), yellow (xanthophylls), or even purple or red (anthocyanins).
Once the pigment has absorbed the light energy, the light reaction takes place. This is the first of the two steps that make up photosynthesis. In this step, the light energy is converted into chemical energy, meaning that it can now be stored in preparation for the other half of the reaction (the dark reaction). It is stored in the form of compounds called ATP and NADPH.
The light reaction takes place inside the chloroplast, in a double membrane fold called the thylakoid. On the surface of this microscopic tissue, in every cell in every leaf of every green plant on Earth, biology is working away to ensure that the sun's light will be converted into food for the plant, and, by natural extrapolation, the animal kingdom as well.