Each vertebrae has a thick drum-shaped area in front called a vertebral body. Between the vertebrae are spaces that allow nerves (nerve roots) to go from the spinal cord to other parts of the body.
In anterolisthesis, the upper vertebral body is positioned abnormally compared to the vertebral body below it. More specifically, the upper vertebral body slips forward on the one below.
The amount of slippage is graded on a scale from 1 to 4. Grade 1 is mild (20% slippage), while grade 4 is severe (100% slippage).