Think of ISO as a sift or a screen. A really low ISO number is like a very fine mesh screen. You can pour liquid like chicken stock through a fine screen and it will capture all the "junk" in the screen, but it takes longer.
A really high ISO number is like a sift with really big holes. The water flows right through but so does a lot of the trash.
The high ISO can capture more light and therefore works in dim light, but the "trash" means the image isn't as clean and sharp.
If you have a lot of light, then use the best sift you have, and that means ISO 100. It's produce the cleanest image.
