Understanding your lens aperture or "F-Stop" is an important factor in creating different looks to your images, as well as controlling light. Below is a diagram that helps to explain it.
The lens represented in the diagram is a Nikkor 50mm f/1.4G. When no blades are visible, the lens is at its maxium aperture. This is when it lets in the most light, giving us the fastest shutter speed possible for correct exposure. It helps facilitate focusing too. When used at maximum aperture, the lens has the smallest depth of field and so only a small area from front to back is in focus (It is important to mention that distance to subject also greatly affects depth of field but, for now lets concentrate on the aperture). Using a wide aperture to create a shallow depth of field can be particularly useful if we wish to draw emphasis to a person or object by isolating them from their surroundings.
The lens is also at its softest when used wide open, meaning it's not rendering as much detail as is possible. As we stop the lens down further, we let less and less light in. We also increase the depth of field, bringing more of the scene into focus. The sharpness of the image also increases until we hit the optimum or "sweet spot". Beyond this we continue to bring more of the background and foreground of the image into view and decrease the amount of light entering the camera. Sharpness does suffer as a result due to diffraction.
The 3 coloured bars in the diagram show different sets of aperture values. Whole stops of light are shown on the green bar. Half stops are listed on the yellow bar. If you're familiar with f/stops but don't recognise these particular numbers, it's because they aren't used that often. Generally it's only the whole stops and third stops which cameras utilize by default, for greater control over exposure and it's worth just concentrating on those.
Observing the numbers closely we can see a pattern emerging. In the green row, the numbers are doubled every other stop. f/1.4 to f/2.8 is two stops of light, just as f/2 to f/4 is two stops of light. This is not the same as shutter speed or ISO, whereby a whole stop is double or half the current value. For example adjusting ISO from 100 to 200 is a whole stop of light, or increasing shutter speed from 1/250 sec to 1/500 sec. Make sure you memorize this, as you can quickly run into exposure difficulties it you treat aperture sequences like those of shutter and ISO.
F/stops form one part of the Exposure Triangle. It is important that you not only understand the role of the aperture but, how it interacts with both shutter speed and ISO. As aperture is responsible for controlling the amount of light entering the sensor to control exposure from manual speedlites and studio strobes, it is equally important to understand F/stops for flash photography.