To put it simply, aperture is the size of the opening in the lens when a picture is taken. It is one of the important elements of exposure triangle apart from ISO and shutter speed. Aperture size is usually depicted in f-numbers i.e. f/36, f/22, f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2, f/1.8.
Aperture and Depth of Field:
The size of aperture has direct impact on Depth of Field (DOF). DOF is the area of sharpness between foreground and background objects in focus.
Large aperture (i.e. a smaller f number and large opening of the lens ) will decrease DOF and small aperture (i.e. larger f numbers and small opening of the lens ) will give you larger DOF. It may appear confusing in the begining , a simple way to remember is that small f number means small DOF and large f number large DOF.
Small DOF (a small f-number) means only subject will be in focus and background of the image will be blurred. This is useful for a portrait or macro photograph where the subject in the foreground should be in focus. Please see first picture with aperture f/5.6 where the flower in the foreground is in focus and the background is blurred.
Larger DOF (a large f-number) means subject as well as background will be in focus. This is useful for landscape photography. Please see picture two with aperture f/36 where the flower in the foreground as well as the background are in focus.
However, Aperture cannot be used in isolation and has to be used keeping in mind other elements of exposure i.e. ISO and shutter speed. To start with, one can use Aperture only setting and the camera takes care of Shutter speed and ISO settings before graduating to complete manual mode.