The exposure triangle is a useful way of describing the relationship between the three aspects of exposure. Each corner of the triangle represents one of the three variables, aperture, shutter speed and ISO. Adjusting just one of these will make the photo darker or brighter and will change the appearance of the photo based on what you have changed. E.g. using a longer shutter speed will introduce motion blur to your photo but also make the photo brighter (increased exposure) due to more light hitting the sensor. The easiest way to understand it is to see the picture.