The exposure triangle refers to the three elements that work together to affect the exposure of an image. These are the aperture, the shutter speed, and the ISO. The correct combination of these three elements produces a technically good/strong image.
Aperture
The aperture is the size of the opening in the lens when a picture is taken. Using a larger aperture when taking a photograph, allows more light to enter the lens. Aperture is measured in f-stops. The smaller the aperture, the larger the f-stop number. In other words, ƒ2.8 is a much larger aperture than ƒ22.
Shutter Speed
Shutter speed refers simply to the amount of time the shutter is open when shooting an image or the length of time the image sensor is exposed to the scene being captured. Shutter speed is measured in seconds or fractions of a second. The higher the denominator, the faster the shutter speed. For example, 1/1000 is faster than 1/30.
ISO
ISO measures the light sensitivity of the image sensor. The lower the ISO number, the less sensitive the image sensor is to light and the finer the grain in the shot. Higher ISO settings are generally used in darker environments to get faster shutter speeds.