Photosynthesis is the process by which plants absorb and convert light energy from the sun into chemical energy they need to grow and develop. A simple way to write this chemical reaction is:
Carbon Dioxide + water + sunlight = Carbohydrate (sugar molecules) and Oxygen.
The process can be broken down into two stages:
Light reactions (light dependent- require sunlight, known as Photons. Produce organic chemical energy)
Dark reactions (light independent- do not require sunlight. Carries out a process known as Carbon Fixation, where carbon dioxide is converted to sugar using the energy produced above ).
Photosynthesis is vital for organisms requiring oxygen (eg. Humans). Without this process life on earth would come to a halt- not just in the direct production of oxygen for us to breathe, but also because the evolution of the food chain is such that we are dependent on survival by eating animals which are themselves dependent on eating plants.