Try here for some basic intro:
http://www.photoradar.com/category/techniques/basic-photography-skills?page=1
But in basic terms...
The aperture (Fnumber / F-Stop) "controls" one aspect of how much light enters the camera - smaller the number, the more light. It also affects how much appears to be in focus, the bigger the number the greater the depth of field (more appears in focus).
All lenses have an aperture that gives optimum optical performance, generally this is around F8 - F11.
The other factor that controls how much light enters the camera is shutter speed - faster shutter speed less light.
You can achieve the same exposure with different combinations of shutter speed and aperture. On your 400D, put it in P mode (Program) focus, and then move the control wheel by the shutter release back and forth. The exposure will be maintained but the aperture and shutter speed will change. The guides in the link above will start to explain the benefits of adjusting these away from the initial value. Using the other modes M, Av, Tv will make more sense once you get to grips with the basics.