What is Aperture
In simplest terms, the aperture of a camera is just the hole through which light enters the camera. By varying the size of the hole we can control how much light hits the sensor or the film. When the hole is large, this is a large or wide aperture, and more light is let in. If the hole light is entering through is small, this is called a small aperture and less light is reaching the sensor/film. Rather than using subjective words like "smaller", "wider", "big hole", etc. aperture in photography is referred to by a "F number". Technically speaking the F number is the ratio of the focal length of a specific lens to the diameter of the entrance pupil. Different sizes of lenses will have different physical sizes of openings and what an F number does is standardize this so that F/4 gives the same amount of luminance (brightness) in your image regardless of the lenses focal length. If your eyes just glazed over after reading the last two sentences, all you really need to take away from this is that a large "F number" (F/16, F/32, etc.) gives you a small aperture (small hole) and lets less light in. Similarly, a small "F number" (F/1.4, F/2.0, etc.) gives you a wide aperture (large hole) and lets more light in.
The term "F stop" in photography often refers to the standard F stop scale which modern lenses use for their aperture values. These values are related to the sequence of 1/2 squared and give F numbers: F/1, F/1.4, F/2, F/2.8, F/4, F/5.6, F/8, F/11, F/16, F/22, ... . When you change the aperture value on your camera from a smaller number to the next larger number in the sequence (F/8 to F/11 for example) you are changing the aperture by one F stop and consequently reducing the amount of light entering the camera by a half. Similarly, when you reduce the F number by one F stop (from F/4 to F/2.8 for example) you double the amount of light hitting the sensor/film. Increasing the F stop, thereby making the hole smaller and letting less light in, is known as stopping down the aperture. Similarly, decreasing the F stop, thereby making the hole bigger and letting more light in, is known as opening the aperture.