The aperture is the "hole" through which light enters the camera to make an exposure on the film or digital sensor.

The smaller the opening, the less light gets in. Aperture settings are labeled as "f-stops", and the larger the f-stop number, the smaller the aperture opening!

The smaller the aperture, the longer the shutter speed you will need to get a proper exposure (ignoring ISO settings or Film speeds, for now).

The problem with longer shutter speeds is that moving objects become blurred, and if the camera is being handheld (not mounted on a tripod) then the shaking can ruin the shot with blurs.

(more to come...)