Aperture is just a fancy way of saying "hole." The lens has a hole in it so light can pass through. You can make the hole larger or smaller depending on your needs. For instance, if you're in a dark environment, you can make the hole larger so more light can enter. If you are outdoors on a bright day, make the hole smaller to prevent too much light from getting in. That's it really: a larger aperture gets you a brighter image.