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The exposure triangle is a graphic representation that summarizes the physical laws that influence light, and that come into play when capturing a photograph with a camera.
Explained in very simple words, the ISO represents the ability of your camera sensor to capture light, the higher the number, the brighter the photo will be.
The shutter speed is the speed with which the shutter of your camera closes when you press the shutter, with a higher speed you can capture a static image, for example the wings of a bird, with a slow speed you will get a photo in " movement" such as those beautiful photos in which the water of a waterfall looks like silk.
The diaphragm opening refers to how open the diaphragm of your camera is when you take the photo; a wider aperture (smaller number) will allow you to focus on a smaller area of the image while a narrower aperture (larger number) will focus on a larger area of the photo.