To put this simply, each system, photosynthesis and respiration, depends on the other with only one input and one output. The input being energy from the sun and the output being waste energy in the form of heat (like temperature, or light, or infrared radiation).
The reactants (like the ingredients) in photosynthesis are energy in the form of light, carbon dioxide, and water. After going through the process of photosynthesis, the products (what is made by photosynthesis) are Oxygen and sugar.
The reactants for respiration are sugar and oxygen which match up perfectly with the products from photosynthesis. The products from respiration are carbon dioxide, water, and energy, which (except for the amount of energy) matches the reactants for photosynthesis. The energy from respiration is not the same amount as went into photosynthesis, meaning, there's a net loss of energy with this cycle. That is why sunlight is needed to 're-energize' the cycle.