As mentioned before, there are three key components to exposure. Aperture, ISO, and shutter speed.
Aperture is the size of the opening (allowing more or less light in to the sensor)
ISO is the sensitivity of the sensor to light
Shutter speed is how long the shutter remains open (mirror up, mirror down *CLICK*)
A typical analogy is that of a hose. Aperture is the size of the opening of the hose, ISO is the quality of the hose and shutter speed is how long the hose is left open to let water in/out.
Each variable works together in creating the exposure, but each has its pros and cons. In this post, we're going to focus on aperture.
Aperture, as mentioned before, is the size of the opening. This variable is typically measured in f/ stops and is limited by the lens that you use. A smaller f/ number, means a larger opening. Counter-intuitive, I know. Each lens has its own "maximum aperture" which is the smallest f/ number that it can be stopped down to. The lower the f/ number, the more expensive the lens typically is.
There are two types of lenses: fixed aperture and variable aperture. Fixed aperture lenses have one f/ number and remain at that f/ number throughout the zoom range. Variable aperture lenses are found typically on lower-end zooms / kit lenses. These have a maximum aperture that changes as the focal length/zoom changes. (e.g. a 18-55 f/3.5-5.6 lens will have a maximum aperture of f/3.5 at 18mm and f/5.6 at 55mm. The max aperture at 19-54mm will be somewhere within 3.5-5.6)
Enough about the technical stuff. Let's get to the nitty gritty.
Aperture affects how much light comes in and depth of field.
A bigger aperture (smaller f/ number) allows more light to enter. Allowing you to use a faster shutter to capture an image. Therefore, bigger aperture = brighter, smaller aperture = darker.
Which raises the question, "Why would I ever want to lower my aperture? Don't I want a lot of light?" leading us to the next effect that aperture has on the image: depth of field.
Depth of field refers to the "depth" of your image. Ever see portraits where eyes are in focus and ears are out of focus? The blur is caused by a large aperture (small f/ number). A larger aperture will reduce your depth of field (make more shallow). f/1.2-f/2.8 will give you great subject isolation, but are difficult to use at times because a slight error will cause your image to be out of focus, because the depth of field (dof) is so thin/shallow. For group shots, you'll typically want to use an aperture of f/4 or lower to make sure everybody's in focus and for landscapes you'll want to use f/8-f/11 to ensure that EVERYTHING remains in focus.