Dynamic range is basically the difference between the brightest and the darkest areas in a scene. The greater the difference, the greater the dynamic range the scene is said to have. A scene with high dynamic range is also called a scene with high contrast.
Truth of the matter is that film and digital cameras have their limitations when capturing scenes with high dynamic range or high contrast. The challenge lies in their ability to record the brightest details and the darkest details of the scene at the same time, without any detail clipping.