A subduction zone occurs when one tectonic plate slides beneath another plate. One plate sinks while the other rise. The classic example this is the Pacific North West. Here a portion of the Pacific Plate is moving underneath the North American Plate. The effect is fresh new mountain ranges and lots of volcanoes. You can demonstrate the same process by pushing a shovel into the ground and lifting up a bit of dirt. The uplift in dirt is a reasonable approximation of what is occurring beneath the surface of the earth.