This is a chemical reaction that happens in the green pigment you see on plants, called chlorophyll. The chlorophyll is able to make food for the plant using light to produce starch (a bit like the stuff in potatoes, rice and wheat). Plants also produce oxygen as a waste product of this process, which is why they are so important for humans as plants produce the oxygen that we breathe.
Photosynthesis is the process through green plants make their own food from sunlight. They are able to do this because they contain the green pigment called chlorophyll and in the process they use up carbon dioxide and expel oxygen. Animals cannot do this because they lack chlorophyll.
They have already explained it.
I’d only add that it is the process that supplies most of the energy necessary for all life on Earth, because it transforms the carbon that is in the atmosphere in form of CO2, to chemical compounds like sugars that animals can eat and then use to produce the energy they need. Without photosynthesis, life on earth would be impossible.
Yes I agree with all the other answers – photosynthesis is the plants’ way of feeding itself, using sunlight, water and carbon dioxide. Usually the more carbon dioxide in the atmosphere the more plants photosynthesize which is one reason why people are keen to plant lots more trees to help reduce the amount of carbon dioxide in the atmosphere that is currently contributing to major changes in our climate.