Aperture is one of the three main controls you can use when you are taking a picture. Along with shutter speed and ISO, aperture controls how light will hit the sensor (OK, old schoolers – hit the film).
In very simple words, aperture is the “size” of the hole the light goes through when it passes the lens. So large apertures will let more light go through then small apertures. Going back to the pipe allegory analogy, we can see the following: If we use smaller aperture, then to keep our exposure unchanged we have to use longer shutter speed, or higher ISO.