While you're fresh we'll tackle the math stuff first. If you skip it I can guarantee you'll be lost further on. In fact you'll probably have to go over this first part several times. Believe me, you won't be the first, nor the last to do so but it's part of the essential knowledge you need to have for making creative exposure adjustments.
Inside the lens is a mechanism called a diaphragm that adjusts the size of the lens opening (aperture), larger or smaller. Controlling that lens opening is the first step in controlling the exposure. The photograph on the left shows a 50 mm lens aperture set to f/2.8 (#1) and then adjusted to an aperture of f/16 (#2).