A conveyor belt is a device that presents a continually moving surface that is designed to move objects from one location to another.
Your basic conveyor belt is a long loop of rubber or plastic (usually combined with steel for strength, just like tire treads), that is wrapped around a set of motorized rollers that cause the surface of the belt to move forward or backwards.
Conveyor belts are most often used in assembly lines, where they move the unfinished product from worker to worker until it is completed. Other uses for conveyor belts include moving sidewalks, escalators, and other people movers (yes escalators are conveyor belts). If you have never seen a conveyor belt try visiting your local grocery store, most check out lanes have them now to feed a constant stream of merchandise towards the cashier to expedite the checkout process. Any grocery store system made in the last twenty years should also include a laser beam near the scanner that causes the belt to stop moving when merchandise reaches it. This keeps the belt from just tossing all those pizza rolls and cases of Burple onto the ground.
The world's longest conveyor belt is over sixty miles long (the Fosbucraa conveyor belt). This absolutely massive outdoor belt is located in the Sahara desert, where it transports phosphate from the mines in Bu Craa to a point on the coast near Laayoune. This belt is so large that it is easily visible in satellite photos, even relatively low resolution ones. This particular belt was attacked by the Polisario Front back in 1976, forcing a halt to phosphate mining in the area for several years
Conveyor belt levels are also a staple of most platform video games. They usually involve the hero dodging obstacles on a belt that moves ever forward. A few early games that contain conveyor belts include Contra and Megaman 2. Many times the conveyor belt levels are frustratingly difficult when compared to the more static levels. Some games also include mine cart levels, which are nearly the exact same thing, just at a different rate of speed, and with a different graphic appearance. The end result is still the same (you cannot stop moving forward).