Spondylolisthesis is a condition in which a bone (vertebra) in the lower part of
the spine slips forward and onto a bone below it.

This is an x-ray to help illustrate this condition caused by bone slipage:

Spondylolisthesis may vary from mild to severe. Some fortunate people with this condtition may have no symptoms; however, most experience some level of pain.
The condition can produce increased lordosis (swayback), but in later stages may result in kyphosis (roundback) as the upper spine falls off the lower spine.

The arrow indicates the one that has slipped onto the bone below it.
Symptoms may include:

Nerve damage (leg weakness or changes in sensation) may result from pressure on nerve roots and may cause pain radiating down the legs.
Complications that can occur with Spondylolisthesis