The use of aperture within a shot allows focus to shift from clarity of items it the for ground with blurring behind, to fully focused crisp shots.
When thinking about aperture the easiest thing to compare it to is the human eye, in particular the pupil which controls the amount of light entering the eye.
When the lens if fully open, the items in the for ground are cleared than those in the back.
When the lens is narrow, the light is more focused and the image is clearer.
The higher the number on the F scale the smaller the aperture.
The two photos below will demonstrate this.
The first using a large aperture:
With this shot the flower at the front is crystal clear where as the one behind is blurred.
The second image is with a small aperture:
This shot shows clear detail on the purple flower and leaves