Exposure Triangle is the relationship between three Pillars of Photography i.e ISO, SHUTTER SPEED and APERTURE. Beyond Automatic mode in the camera you need to play with these settings iso, aperture and shutter speed. The relationship between these three pillars which is confined by the rule is called Exposure Triangle.
Playing with three variables is really difficult unless you are clear about them. All the three pillars Iso, Aperture and Shutter Speed can determine the exposure of a photograph.
ISO is the sensation of the camera to the available source of light, so while looking after iso you must also consider shutter speed and Aperture. For example, if you have enough light at your surrounding and you have set your iso to 100, it is equally important to balance the Shutter Speed. ie, if your shutter is open for long period of time then your image will be overexposed being more brighter. In the same you should set your aperture accordingly preventing the image to be overexposed or underexposed.
If you are trying to capture the fast moving object in medium available light with higher shutter speed, it is equally important to to balance aperture and iso. In this condition you must set your Aperture number low which allows you to pass the light more making image brighter and more focused. And similarly you should also higher the ISO level to some extent to make image more brighter as per requirement though you might experience some noise in image.
Hence it is equally important to balance all the three factors ISO, APERTURE and SHUTTER SPEED for the desired result in photography making EXPOSURE TRIANGLE to work out the most.