A scheme showing examples of photochemical reactions in nature
Photochemistry is the study of chemical reactions with light. There are many natural examples. One is photosynthesis, how plants make oxygen from carbon dioxide and water. Another is making vitamin D in the body.
Many times reactions go with heat: as you heat molecules, they start to move around and so hit other molecules which can then react. What molecules need is just some activation energy, a little energy to start the reaction. In the case of photochemistry, this energy is given by the light. A molecule or an atom absorbs at a certain wavelength. If light is given at this wavelength then the molecule or atom becomes excited. It is then easier for it to react.