The aperture is the adjustable opening in the lens that allows light in to strike the image sensor. Apertures are sometimes referred to as f-stops.
The most common settings are f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22. The larger numbers represent smaller-diameter openings, and these smaller apertures allow less light into the camera to make an exposure.
Making a one-stop shift in aperture will halve or double the amount of light that strikes the sensor. For example, if your aperture is f/8 and you move to f/11, you will reduce the light by half. Moving from f/11 to f/8 will double the amount of light that reaches the sensor.
The f-stop also controls the amount of the scene, from front to back, that will appear in focus. A wide aperture like f/2.8 allows for a very shallow focus depth, while a narrow aperture like f/22 allows for much more of the scene to appear in sharp focus. This phenomenon is referred to as depth of field, and it is an important tool in creating effective images.
Depth of field is also affected by the focal length of the lens. Telephoto lenses compress the image and make a shallower (narrow) depth of field with less of the image in focus. A wide angle lens will show a wider view and give more depth of field.