That's the right idea. Just take a photo, look at it and decide what you would change, and remember that next time. Content and timing are far more important than composition or exposure anyway. Some of my favorite photos are blurry, dark or blown out.
Stopping down the lens = closing the iris slightly, also achieved by manipulating other functions of the camera such as shutter speed and exposure compensation. In simple terms, make the photo darker.
Layering in Photoshop = take two images, place one on top of the other and blend them together to make one better photo.
Exposure limit = the range of contrast the camera can handle before detail is lost. Notice in the photo above that the sky is blown out (too bright) on the left. If you lowered the exposure on teh camera to make it right, the dark areas would disappear. If the camera had a higher exposure limit then this would not happen as severely. Also known as contrast ratio on TVs and computer monitors.