Aperture, shutter speed, and ISO work together to produce the exposure in photography. A correct exposure makes all the difference. If your image is underexposed, it will be too dark. In the case of an underexposed image, you can either open up your aperture, slow down your shutter speed, or increase your ISO (or all three if needed). If your image is overexposed, it will be too light. In the case of an overexposed image, you can either use a smaller aperture (higher number), increase the shutter speed, or decrease the ISO (or all three if needed). This video by Tony Northrup does an excellent job of explaining how the three elements work together.