The primary purpose of each of the legs in the Exposure Triangle is to obtain a properly-exposed image. Each of the legs can be adjusted to obtain a good exposure. One has a great deal of freedom in choosing these settings: one can move up one leg if ones moves down another and still get a well-exposed image.
Each part of the Exposure Triangle can be measured in something called “stops.” One stop changes the exposure by a factor of two, brighter or darker. If you change the exposure by two stops, you have changed the brightness by two factors of two, that is, by a factor of 4. If you change the exposure by three stops, you have changed the brightness by a factor of 8 (2x2x2).
The reason why we use the word “stop” is obscure, but it will be explained (if you care) in the aperture section.
Here is a cheat sheet for the use of the term “stop”.
|-3 stops||Eight times darker|
|-2 stops||Four times darker|
|-1 stop||Two times darker|
|0 stops||Exposed properly|
|+1 stop||Two times lighter|
|+2 stops||Four times lighter|
|+3 stops||Eight times lighter|
Next: Shutter Speed