ISO

ISO controls how much gain is applied to the signal coming off the sensor. Raising it makes a dark room usable and amplifies noise along with the picture, which shows up as grain in the shadows and coloured speckle across skin. Most cameras have one or two native values where the sensor is cleanest. Park on those. Order of operations matters as well: add light, open the aperture, set the shutter to roughly double your frame rate, and only then reach for ISO.