Aperture

f2.8

f18

The human eyes' pupil can be closely related to aperature.
The smaller the aperture, the less light enters the lens. The higher the aperture, the more light enters the lens.

If you have a small aperture then every object in your photo will become clear so you can see everything in the background and foreground. If your aperture is higher then your "pupil" opens up more and takes in more light also causes the foreground to be in focus and the background blurred.