The aperture on your camera or better still the aperture on your lens is what controls the amount of light coming into the camera. It is one of three things that control the exposure, the other two being shutter speed and ISO but more about those two in other tutorials.
The interesting thing about the aperture is not only does it control the amount of light coming through the lens but it also controls what is in focus. A small f-stop, that is when the aperture is open wide letting a lot of light into the camera, means that whatever you have focussed on will not have much focussed in front of behind the subject. However, I higher f-stop means that less light is coming into the camera but a lot more will be in focus in front of and behind the subject you are focussed on.
Sometimes you only want the subject to be sharply focussed and other times when you want a lot in focus. A portrait for example may just have the subject in focus whereas a landscape may need everything to be in focus.