Only recently when I knew about this. :)
It's a must read especially for photography beginners like me.
That aperture is the one responsible for the focus of your subject when shooting a photograph. Technically speaking,
aperture is the size of the lens' opening when taking pictures. Aperture is measured by what we call the "f stops". They are what we refer to as f/4, f/5.6 and so on..
Keep this in mind: large apertures are indicated by f/stop smaller numbers; small apertures have bigger f stop numbers. Large aperture is where large amount of light gets through while small aperture is where little amount of light gets through the lens.