It's used in many words related to light, such as:
* Photosynthesis: The process plants use to convert light energy into chemical energy.
* Photoelectric effect: The emission of electrons from a metal when light shines on it.
* Photograph: An image created by capturing light on a sensitive surface.
* Photovoltaic: Converting light directly into electricity.
So, whenever you see "photos" in a word, it's a good bet that it has something to do with light!