I used to be a beekeeper. These days we let other beekeepers use our property, but bees will always be connected to ISO in my world. Here is a quick way to remember how ISO connects to photography. Whenever you think of ISO, think of these words:
IN SEARCH OF light
When I used to teach high schoolers about ISO, I would have them imagine bees heading out from a hive. Instead of collecting pollen and nectar, they collected LIGHT. The more bees that were out capturing light, the more light they brought back to he hive. A few bees would capture a small amount of light, but a lot of bees would capture a lot of light.
100 ISO = 100 bees IN SEARCH OF light
10,000 ISO = 10,000 bee IN SEARCH OF light
Bees can make a lot of NOISE. 100 bees make some noise, but 10,000 bees make a lot of NOISE! The more bees you have out capturing light, the noisier it gets. All that buzzing makes some static and interference too. Static and interference make noise.
A clear and noiseless image begins with adequate light for the exposure you want to make. Adequate light means your ISO will be at the bottom range for your camera--generally in the 50-100 range (less bees). You might get there with a bright, sunny day or you might use off camera flash or constant lighting.
Raising the ISO increases the light-collection ability of your camera (more bees). A very high ISO may make the image possible, but may have some grain (noise). You may have to raise your ISO when light is dim, when you want to close your aperture down for deeper depth of field, or when you want to have a fast shutter speed. All of these situations may require you to raise the ISO to make your image. The trade-off is the potential for some noise.
The cool thing about all of this is that Sony cameras ROCK when it comes to collecting light with higher ISO numbers, but they also manage all that buzzing static (noise) really well too! My first camera with Sony was the original Sony a7s. I took portraits with tea light candles and the rest is history!
Tip: Map your ISO adjustment to the wheel on the back of your camera. I find this position makes it very easy to tweak ISO without ever having to take my eye away from the viewfinder.