ISO is how sensitive the camera is to light. The ISO numbers are to tell the camera how much light to let into the frame of the shot when the picture is taken. The numbers are 50, 100, 200, 400, 800, 1600. The lower the number the crisper the image is the higher the number the grainier the image is. A high ISO is good for dark places where you aperture doesn’t allow enough light into the camera e.g. cathedral but the downside is the picture starts to become more grainy.
Explanation Of ISO.