Exposure is the amount of light striking the sensor.
The amount of light is controlled by using three controls.
ISO is a measure of how sensitive the sensor is to light.
The lens opening is an iris-like device in the lens.
By changing the size of the lens opening, the intensity of the light reaching the sensor is varied.
Lens opening is also known as aperture, f/stop, and rarely, diaphragm.
When the shutter opens, the duration of the light reaching the sensor is controlled.
Let's look at ISO, lens opening, and shutter speed.