Plants absorb sunlight and turn that energy into food; the process is known as photosynthesis.
This is a compound word made up of photo (which means "light") and synthesis (which means "to put together"). Think of a synthesizer, which puts together different sounds to produce music. A plant uses light to put together chemical compounds and turn them into carbohydrates: food. Although the word can be used to describe any process that uses light to connect compounds, the word is almost always used to describe a plant's method of making food.