Refer the picture.
The subject is in focus and the blurred background separates it and invites our attention to the person. The magical blur in the background is called Bokeh. The blurriness increases as we open up the aperture.
• Case A:
Here in this photograph below, the focus extends from the front of the face to the back of the head. This distance between point A and B is called depth of field.
The subject in the middle is in focus and the subjects in the front and at the back are not. The picture is taken at an F stop number F 1.4 which gives a very shallow depth of field.
Depth of field at F 1.4
• Case B:
In this picture the subject in the foreground and in the background both are in focus. So the depth of field has increased in this case making both subjects in focus. This picture is taken at a depth of field of F/16 that is at a small aperture which will increase the depth of field.
Depth of field at F16
Hence Aperture helps to control the light entering the camera to achieve a normal exposure. It also helps to change the look of the image aesthetically.