Inside every camera lens is an opening called the aperture which works in exactly the same way as the pupil in a human eye. The aperture changes diameter from a wide aperture which lets in lots of light, to a narrow aperture which lets in less light.
Aperture diameter is measured by an “f-number”. Typical values are f2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32. Somewhat confusingly, a smaller f-number represents a wider aperture and therefore more light, and a higher f-number is a narrower aperture and less light. (For the mathematically minded this is because f-number = focal length / aperture diameter).
A wider aperture/lower f-number (left) lets in more light, and vice versa.
An important thing to grasp is that these f-numbers are spaced one “f-stop” apart. By moving one f-stop you either double or halve the amount of light the aperture admits, meaning you need to either halve or double the shutter speed to keep a constant exposure.
THE HOSEPIPE AND BUCKET ANALOGY
Before we get too bogged down in technical terms, let me tell you a simple analogy that I learned years ago, and which really helped to clarify in my mind how aperture diameter and shutter speed combine to give exposure.