ISO Stands for International Standards Organization (which never existed BTW).
The ISO you choose determines the sensitivity of your camera’s sensor. This effects your camera’s exposure.
Back in the day we bought 100 ISO film for bright outdoor days, 400 was for cloudy or modestly dark situations and 800 was for indoor and dark places.
The video and cheat sheet below explain ISO very well. Once you have mastered what ISO settings to use and when, you have made a significant step towards better photography. If you use Auto ISO, now is the time to begin learning this very helpful camera feature.