What is Mild retrolisthesis of the L1 and 2 level and L4 and 5 level

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2278018

Retrolisthesis (backwards slippage of one vertebral body on another) has historically been regarded as an incidental finding, one which doesn't cause any symptoms, and is considered to be of little or no clinical significance. Few studies have been done to date and little is known about this condition.