Stenosis means "narrowing" of an opening. In the case of spinal stenosis, the opening for the spinal cord, called the spinal canal, narrows. The result is to put pressure on the spinal cord as it goes through the canal.
Sometimes the openings for the spinal nerves get closed down, too. This is another cause of stenosis. Aging and disc disease are the two biggest factors in this condition. The nearby joints start to thin and form bone spurs. A large ligament in the spine may thicken, causing it to begin to buckle.
All of these changes can cause spinal stenosis. In a few people, stenosis was present at birth. This is called congenital stenosis. The changes that occur with aging are called acquired stenosis.