"The Exposure Triangle" is a catchy phrase meant to encompass the three factors which affect the exposure of a photograph of a scene with a given amount of light. It's often given to new photographers as a learning aid.
The three factors are:
Shutter — how long you let light in Aperture — how large of an opening you let light through (how much at once) ISO — how sensitive your film or sensor is to light
Each factor is interchangeable in terms of exposure, so that a decrease or increase in one factor must be met by the same amount of change in another.
The problem with the phrase "exposure triangle" is that the relationship between these three factors does not actually share any of the properties of a triangle other than "threeness". That makes it a bad analogy, which can introduce more confusion than necessary, as new photographers attempt to reason from the information they've learned.
The best explanation I have found for the reality of exposure can be found in this video.