The meter understands what the ISO numbers mean. Your aperture is the series of metal blades in your lens that control the amount of light. The number is a ratio of the focal length of the lens to the actual diameter of the opening. Because the number is a ratio, the higher the f stop, such as 16, the smaller the size of the opening Conversely the smaller the number, such as f 2, the larger the opening. This concept is quite confusing to some so allow me to repeat it.
Large number equals small opening and small number equals large opening.
There is a specific reason for this progression. As we move from a lower to a higher number such as from 2.8 to 4, we have let in less light. In this case, we let in half the amount of light. A move in the opposite direction, from 4 to 2.8, lets in twice the light. Let's set this concept aside for a second and explore shutter speed, our last variable.