'Aperture' refers to the size of the iris (hole) within the lens. The bigger the iris, the more light enters the lens. 'Aperture' is measured in F-stops. A number like F/1.8 or F/2.1 means a wider aperture. However, a number like F/16 would be a smaller aperture.
A wider aperture (a low F-stop) means less of the photograph is in focus. I also means a faster shutter speed can be used.
The diagram below shows how aperture works.
My Experimentation
The photographs below show different aperture settings on the same image focus (the leaf). Starting from F/1.4 and ending with F/8.