The first aspect of the f/stop, or aperture, is fairly straightforward: The lower the number, the more light you’re letting come through the lens.
Think of the aperture like the pupil of your eye. A low number f/stop, (i.e. f/2.8) lets more light through the lens, a dilated pupil, than any higher f/stop number. As the f/stop number goes up, less light is allowed through the lens, a small pupil.
Take out your camera and push only halfway down on the button to take a picture. Do you see a number on the screen with a decimal? Like 2.8? It might even have an f in front of the number, like f8 or f/5.6. That’s your f/stop! Yeah, you found your f/stop, but now what?
Next in the series, demystifying depth-of-field!