Aperture

This is a diagram of different apertures. F2 the shutter is wide open, a large aperure. Then on the other end F16, a small aperture, the shutter is very small.

F.32

F.29

F.20

F.14

F.10

F7.1

F5.6

The aperture controls the amount of light that reaches the sensor on a digital camera. An aperture acts much like the pupil of an eye. It opens wider as light decreases to let in more available light. It gets smaller when light increases to reduce the amount of light entering the eye.
The wider you set the aperture, the faster your shutter speed should be. If you leave a longer shutter speed on a wide aperture, the chances are your image will be over exposed. Leting too much light into the camera, will over exposed an image. Wider apertures create a smaller depth of field, meaning less of the image will be in focus.
Alternatively, if you have a small aperture, you will need a longer shutter speed, to allow enough light into the camera to expose the image. Smaller apertures have a much largers depth of field, meaning most, if not all of the image will be in focus. These types of photos would require a tripod, and or shutter release, to ensure no camera shake, dew to the longer shutter speed.
Apertures are decribed as F stops. The larger the aperture the smaller the number. So F.5 would be a large aperture, with a small depth of field. F.22 would be a small aperture, with a large depth of field.