We've looked at how lens opening and shutter speed are used to control the amount of light reaching the sensor.
How do we measure the amount of light?
Your camera contains a light meter for that purpose.
The light meter measures the amount of light that will strike the sensor.
When you depress the shutter release slightly, the light meter turns on.
You're taking a light reading.
You're measuring the light with the light meter.
Once the amount of light is known, the lens opening and shutter speed can be set.