Aperture is all about how much of the photo is in focus. It is measured in f stops. The smaller the f stop the less that is in focus (as below), the larger the f stop the more that is in focus.
Because I have been wanting to learn about aperture, I have been using the Av setting on my camera. This means that I can set the aperture and the camera will set the shutter speed for me.
I took these photos on my digital camera using the Av setting and with an aperture of F/4.0.