The basic equation is:

Carbon Dioxide + Water + Light Energy –> Glucose + Oxygen

The chemical equation is:

Photosynthesis is the process by which plant cells using the energy from the sun, create food (or glucose). The process is important because we rely on it to obtain oxygen (to breath!) and to obtain energy (to eat!). The importance of it will be explained in further detail in 3).

An easy way to remember what the process entails is to break the term down:

Photo means light.

Synthesis means to make.

So in a nutshell: to make using the energy from light.