Aperture is the diaphragm opening in the lens through which light passes to the image sensor. Cameras that have the option to adjust the aperture provide control over the depth of field in a photograph. When a wide aperture (indicated by a lower f-stop number) is used, more light is passed to the image sensor; this creates a shallow depth of field. By using a narrow aperture (indicated by a higher f-stop number), less light passes to the image sensor and creates a deep depth of field .
IMPORTANT: While aperture is primarily used to control depth-of-field, it can also affect the exposure of a photograph - see the relationship between aperture, shutter speed, and ISO .