Aperture is referred to the lens diaphragm opening inside a photographic lens. The size of the diaphragm opening in a camera lens regulates amount of light passes through onto the camera the moment when the shutter curtain in camera opens during an exposure process.

It very important in photographic element called "depth of field"
The bigger the apertures used, the zone of sharpness is shallower or vice versa i.e. smaller aperture used will has extended depth of field..

Aperture size is usually calibrated in f-numbers or f-stops

Adjust the aperture to change range that is in focus and background blur
Smaller F -   Isolate subject - image
Larger F -  Focuses on all

Larger F Shooting the depth of the scenery.Narrowing down the aperture widens the range that is in focus. (Depth of field becomes deeper)

Smaller F

Putting the subject in sharp focus and blur everything in front of and beyond the subject. Opening the aperture narrows the range that is in focus. (Depth of field becomes shallower.)

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