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 sensor, either correctly, Lesley (under exposure), or more/extra (over exposure). 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 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 sensor through time duration. For how long shutter is opened will allow light to reach sensor for set time duration. In other words if aperture is opening of tap then shutter speed is duration of tap should remain open.
ISO is sensitivity of sensor. If sensor itself is brighter, then less light is required through shutter and aperture, hence smaller aperture and less time duration is 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.