As you already know, both of these conditions affect the spine. Spondylolisthesis is where one vertebral (spine) bone slips in front of another vertebra. In such cases, there’s a forward slip of the the vertebra. But there is also lateral listhesis where one vertebra starts to slide off to the side.
In either case (forward or sideways slide of the bone), the force of gravity shifts over. The spinal muscles try to hold the spine and compensate for that abnormality. These changes often cause mechanical pain in the back. With aging, arthritis in the joints and disc collapse add to the problem.
If the problem of spondylolisthesis occurs in the spine above L1 or L2 (first or second lumbar vertebra), stenosis (a narrowing of the spinal canal) can develop. The shift of the vertebra closes down the canal opening where the spinal cord travels.
Then pressure directly on the spinal cord can cause painful symptoms. But usually this condition affects one of the lower lumbar vertebrae. So it’s the spinal nerve roots that exit the spinal cord below L2 that get pinched or compressed.
Stenosis can also develop as a result of other problems. Basically, anything that makes the spinal canal smaller than it should be contributes to stenosis. That could be bone spurs, thickening of the spinal ligaments, and/or thinning of the disc and subsequent narrowing of the disc spaces. With any of these degenerative (age-related) changes in the spine, painful back and/or leg pain can develop because of the nerve compression.
You may have stenosis from the spondylolisthesis. You could also have spondylolisthesis with stenosis from other contributing factors. You would have to have your surgeon explain the meaning of these two terms in your specific case to fully understand the difference (if there is any).