The Aperture and the Shutter are the yin and yang of photography, these 2 regulate the exposure and work together to affect the total image, and understanding their effect will take you along way down the road of mastering photography.
Aperture, if you look back to lesson one this is the opening that the light come through. On a camera it is normally a set of metal leaves that close in from the out side to leave a small round hole in the centre of the field of view. The wider the hole the more light can get in, if you ever hear of someone shooting wide open, this means that the aperture was at its widest. The wider the aperture the lower the number, so 1.8 is wider than 4.5, these numbers can also be seen on the lens, giving an indication of its speed, a low aperture lens is a faster one, due to the fact that wide open it lets in more light making for a quicker exposure. Due to technical restrictions, the faster lenses cost more to make, so a low aperture on a lens is also an indication of quality.