The definition of the aperture is a measure of how big the lens is open (the openings of the lens) when we take photos.
When we press the shutter button, the hole in the front of the shutter sensor cameras we will be open, well setting the aperture which determine how big a hole is open. The bigger the hole open, the more the amount of light that will get read by the sensor.
Aperture or openings is expressed in units of f-stop. Often we read term openings/aperture 5.6, in the language of photography a more official can be expressed as f/5.6. 
As revealed above, the main function of controlling the aperture is as how big a hole in front of the sensor is exposed. The smaller the f-stop number means the bigger the hole is open (and the more the volume of incoming light) as well as vice versa, the large number f-stop the more small holes open.
So in fact, setting the aperture f/2.8 aperture means far greater than setting f/22. So the size of openings mean growing numbers f and narrow openings mean fewer f numbers.
Depth of Field
Depth of field – DOF, is a measure of how far the field focus in a photograph. Depth of Field (DOF) width means most of the objects of the photo (from the nearest object from the camera to furthest objects) will look sharp and focused. While the narrow DOF (include shallow) means that only the objects on that particular point are sharply while the rest will blur/focus.
To get the DOF wide use small aperture settings, such as the f-22 (the small aperture wider focal). While the narrow DOF to get, use the wider aperture, say be f/2.8.
The concept of Depth of Field will be a lot of useful especially in photography portrait and macro photography, but actually all specialties will need it.
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