Don’t worry about memorizing it, but this acronym stands for International Standards Organization and it’s the third component of the exposure triangle. You may also be familiar with the similar term “ASA” if you shot film. The easiest, and most common (albeit imperfect) way to explain this is that ISO is “the sensitivity of the sensor”. Whereas the aperture number feels backwards, the ISO number is quite logical: the higher the number the more sensitive the sensor and the brighter the image. We’ll go over a more thorough explanation of ISO in a more advanced course but this is really all you need to know for practical photography.