Exposure is determined by three different factors:
- Shutter speed
Shutter speed controls how long the camera allows light to hit the sensor. Longer shutter speeds allow more light to hit the sensor. Shorter shutter speeds let less light onto the sensor.
Aperture controls how wide the lens is open, the wider the aperture more light hits the sensor. Smaller apertures let leases light onto the sensor.
ISO controls how sensitive the sensor is to light.
These are the basics, I'll cover them in more detail in their own sections.