One thing that seems to baffle a lot of new photographers is understanding the term “stop” as it pertains to a measurement of light. We see this word used over and over with regards to shutter speed, aperture, film speed, filters, lighting, and other ways that light is used. You may hear a phrase like “you should expose one side of face 1-2 stops under the lit side of the face and expose the background 1 stop under the main subject”. For many people that’s about like asking them to solve a complex calculus problem.

Source: Cameradojo.com