The exposure triangle is very simple to understand and remember. If there is a word exposure there would be something that takes care of light to reach the sensor, either correctly, Lesley (underexposure), or more/extra (overexposure). The exposure triangle helps how much light is required to take a correctly exposed photograph.
If there is a word triangle, it represents three things. These three elements of the exposure triangle are a) aperture, b) shutter speed and c) ISO.
All these three elements have one thing in common, they all controls exposure. Correct combination of three will allow correct light to reach/enter sensor.
Aperture allows light to reach sensor through opening of blades/diaphragm. It’s like opening of tap which allows more or less water to reach bucket.
Shutter Speed allows light to reach the sensor through time duration. For how long the shutter is opened will allow light to reach the sensor for a set time duration. In other words, if the aperture is opening of the tap then shutter speed is the duration of the tap should remain open.
ISO is the sensitivity of the sensor. If the sensor itself is brighter, then less light is required through shutter and aperture, hence smaller aperture and less time duration are required to expose the subject.
Above was the technical but common role of three elements of exposure triangle (Shutter speed, aperture, ISO)
These three element also have their creative role, Shutter controls motion (freezing or motion blurr), aperture controls depth of field, ISO controls grains on photo.