The larger the aperture in a camera is the hole or opening through which light travels this part of the camera is called an iris. The iris is adjustable in in different increments or stops. Each stop or f number allows half the light in as the previous one. The term f-number or f-stop is used to define the ratio of the focal length to the diameter of the aperture.
Below is a diagram of the different f-stops.
Each f-stop allows half the light in, so to let all the light in with the aperture fully open would be almost the same diameter as the lens.
Normally, I use the f/16 rule when taking photographs. F/16 is the f-stop that is best to use on a sunny day, no clouds with the object in the sun because it lets in just the right amount of light for a balanced image.
Below are some examples of the results from the different f/stops:
Changing the aperture changes the depth of field, the depth in a scene from foreground to background that will be sharp in a photograph. Smaller apertures increase depth of field while larger ones decrease it. For some pictures—for example, a landscape—you may want a smaller aperture for maximum depth of field so that everything from near foreground to distant background is sharp. But perhaps in a portrait you will want a larger aperture to decrease the depth of field so that your subject is sharp but the background is soft and out of focus.
Remember the larger the aperture size, the smaller the aperture. This is a good way to remember what aperture to use when you are out on a photo shoot.
To get a good image with a small aperture, you can increase the ISO.