When we refer to the “aperture” of a lens, we are speaking on an iris, similar to that in the pupil of an eye, that opens or closes, allowing light to enter and strike the optic nerve, sending impulses to the brain. A lens aperture works much the same, except it will allow as much light to enter as we tell it to allow. While our brains can automatically compensate for light or dark (a human with good vision is capable of seeing light as dim as a trillionth of a watt), should we tell the aperture to allow too much light for the ISO we've set the chip to record, the result will be an overexposure that the camera cannot compensate for.