A photon is a particle of electromagnetic radiation. Simply put, it is a single unit of light.
Photons are used to convert light into useful energy, hence Photosynthesis. They are measured in micromoles describing the amount of light coming from a light source. Plants can sense photons using their photoreceptors; the organisms or proteins of the plant that perceive light. The photoreceptors signal various biological changes from each wavelength of light affecting things like structure, growth, biomass, color, taste, aroma and so forth.
The interesting thing about photons is that not all are created equal. Photons vary based on the nanometer. Those that are created by the red portion of the spectrum are typically heavier than those created by the blue portion.