Three controls set how bright an image is. Aperture governs how much light enters, shutter speed governs how long it enters for, and ISO governs how much the resulting signal is amplified. Any given brightness can be reached by countless combinations of the three, which is what makes exposure a creative decision rather than an arithmetic one.
The triangle metaphor captures the trade. Open the aperture two stops and you must halve the shutter duration twice, or drop ISO by four times, to land in the same place. What changes is not the brightness but everything else: depth of field, motion blur, and noise. You are choosing which side effect you want.
In practice most photographers fix whichever variable matters most to the shot and let the others follow. A sports shooter locks a fast shutter and accepts the ISO required. A portrait shooter locks a wide aperture and adjusts shutter to suit. Learning the triangle is really learning to decide what the picture is about before you decide how bright it should be.