Aperture is the f/#'s and it controls how much depth of field is in the picture. DOF = how much of the image is in focus. The higher the f/#, the more will be in focus. However, the higher the number, the more light you need to get proper exposure.
You'll need to read the manual for your camera's model to learn how to change the aperture setting.
Play around with it. It gives you a lot of artistic control.