Photosynthesis is the process by which plants take the energy from the sun and, with carbon dioxide and water, make carbohydrates. The energy is stored in the chemical bonds in these carbohydrate compounds.
Breaking it down
Photo- means "light" and "synthesis" means "putting together" so the word refers to a process of putting together elements, in this case, carbon dioxide and water, with light from the sun.
How is it related to nutrition?
You may think that this has nothing to do with nutrition, but actually it is basic to healthy eating and, in fact, to your survival. It is photosynthesis that produces the carbohydrates that you eat and thus, provides the energy you need to live.
Another way to say this is that this carb-making process allows you to access the energy from the sun for use as energy in your body.
When you eat a green salad, your body is able to metabolize the carbs by breaking the bonds between the atoms that make up each carb molecule. In this way, the stored energy is released for use by your cells. Without the energy from the foods you eat, you would not be able to move, breathe, or have a beating heart.
Even if you don't eat plants, (although I hope you do!) and focus instead on meat, you are still dependent on this process, since the meat you eat was produced by feeding plants to animals. The animal uses the energy in the plant to grow, and they will also store some of the energy in their muscles. Then, when you eat the meat, your body can access the energy, in the form of carbohydrates and protein, through metabolic processes.
Chlorophyll is needed.
Chlorophyll is the pigment that gives plants their green color. It is responsible for initiating the whole process by absorbing light and transferring the energy from that light to other molecules.
Whatever part you eat
Whether you eat the green part of the plant, such as the stems and leaves of celery or spinach, or the fruit or roots of tomato and carrot plants, you are taking advantage of photosynthesis. It is a miraculous process that we mostly take for granted as we enjoy all of the delicious and nutritious foods it produces.