Aperture

The purple areas represent the largest angles at which light could enter the aperture.

When an object is in focus, light rays coming from that point converge at a point on the camera's sensor. If the light rays hit the sensor at slightly different locations, then this object is rendered out of focus.

The lens with the larger aperture has a smaller distance range over which objects reaching the sensor will remain in focus.

A large opening lets in more light to the sensor, but gives a narrower depth of field so less of the image is in focus.

A small opening lets in less light to the sensor, but gives a wider depth of field so more of the image is in focus.

An interesting website -

Aperture and Shutter: Science of the F-Stop

www.picturecorrect.com/tips/aperture-and-shutter-science-of-the-f-stop/

References -

http://www.cambridgeincolour.com/forums/thread1357.htm

http://bohorbfcaphoto.weebly.com/blog/the-examples-of-aperture-iso-and-shutter-speed

http://caseylorzphoto.com/how-to-use-dslr-camera-like-pro/aperture-cheat-sheet