It's where carbohydrates are synthesized from water and carbon dioxide. At least, that's a very common and basic definition of it anyway.
In photosynthesis, organisms (like plants) use sunlight as an energy source and use a catalyst called chlorophyll in the chloroplasts where carbon dioxide and water are used to make glucose (and also oxygen).
Carbon dioxide + water --> glucose + oxygen
OR
6CO2 + 6H2O --> 6C-12H-6O + 6O2