Single lens reflex (SLR) cameras allow user to see the image through the lens by reflecting light into the prism and view-finder using a mirror. When the user is ready to take a picture, the mirror flips up and the shutter door is released to expose film to the light. Since the flipping of the mirror occurs extremely fast, the action causes vibration to the camera, thus a slight image blur.
Some high end cameras provide a feature to lock up the mirror for a few seconds before the shutter door is opened. This mirror lock-up feature allows the vibration to settle before the film is exposed to the light, thereby reducing image blur.
Many low end cameras, such as the Canon Rebel 2000, does not offer this feature. However, in the dark, mirror lock-up can be easily emulated using a simple cardboard. Say you want to expose a scene for 60 seconds. So, set the camera to 'bulb' mode. Hold the cardboard as close to the lens as possible, without actually touching the lens. Open the shutter. Wait two seconds. Remove the card-board and expose for another 60 seconds.