We should think of this like the pupil in your eye. When our eyes need more light, our pupils open up. When there is too much light, our pupils get smaller.
The primary use of aperture is to control the depth of field. How much of your image do you want to be in focus.
In this image, the F stop is 1.4. By choosing a low number (a wide aperture) this enables us to focus on one element in the image.