1) Apertures are expressed in numbers, with the smaller number meaning a larger opening in the lens. A larger lens opening lets in more light, which can increase your shutter speed.
2) Apertures are typically expressed by f/(number), like f/2.8, f/4.0, f/5.6 ,etc.
3) Moving one whole stop in an aperture means doubling or halving the available light to the camera's sensor, depending on which way the stop has moved. f/2.8 would be twice as fast as f/4.0, but only half as fast as f/1.4 (making f/1.4 4 times faster than f/4.0)
So what else is there to aperture? A huge visual impact, that's what!
Let's look at a series of examples:
|Shot at aperture f/5.6|
|Shot at aperture f/8|
|Shot at aperture f/11|
Look at the series of images - notice how every time the aperture gets larger, the background becomes more and more in focus? This is the second "function" of aperture.
The smaller the aperture number, the more light allowed to the camera but the less the background is in focus. Conversely, the higher the aperture number, the less light allowed to the camera but the background is more in focus.
The blur is actually a function of several things - the aperture of the lens during the photo, the size of the sensor (as exampled HERE), the focal length and the distance to the subject.
But, for this tutorial, just remember that all else equal, setting your aperture to lower or higher f-stops can drastically alter how your image looks.