The exposure triangle has 3 elements which are ISO, Aperture, Shutter Speed.
Understanding Exposure With The Exposure Triangle
Shutter speed-the amount of time the shutter is open. Usually measured in fractions of a second.
Fast shutter speed "freezes" motion. Slow shutter speed "blurs" motion.
Aperture-the size of the opening in the lens when a picture is taken. Aperture is measured in f-stops. The lower the f-stop the bigger the opening, the higher the f-stop the smaller the opening.
Depth of Field
Depth of Field is the amount of your shot that will be in focus. Large depth of field means that most of your image will be in focus. Small or shallow depth of field means that only part of the image will be in focus and the rest will be fuzzy.
ISO-the cameras sensitivity to light
If its sunny out your going to use a lower ISO. If its dark out your going to use a higher ISO. In a gym with moving athletes you would use a high ISO.