Aperture is just what it means – a hole in your lens. This is the hole through which light enters the body of the camera, much like our eyes. Now if there is a lot of light then you can reduce the aperture to be very selective of what you want but if the light is low then you have to let as much of it in as possible.
Size of Aperture
The size of aperture is described formally with f-stops. Or f-numbers. These f-stops are inversely related to the size of aperture. So the smaller it is the larger the aperture or opening. Take for instance f/1.4 is actually larger than f/2.0 and way larger than f/8.0.
Depth of Field
Now that size of aperture is understood, you need to learn what depth of field means and how it relates with aperture. Ins simple terms just remember that depth of field refers to sharpness of the image. Using a larger f-number or smaller aperture will help focus both the foreground and background better but a smaller f-number puts the focus firmly on the foreground thus blurring the background up.
Maximums & Minimums
Any camera lens you get always has a limit on the aperture range. The specifications for your lens should mention this. Usually maximum aperture of the lens is of utmost importance since it determines the speed of the lens. Those lenses that have an aperture of f/1.4 or even f/1.2 are said to be very fast lenses since they allow lot more light to go through. Using fast lens is always advisable for low light photography. The minimum though is never a concern since most modern day lenses can go as low as f/16 and needing more than that is actually not necessary.