What does grade 1 anterolisthesis of l5 on s1 secondary to bilateral pars defects mean

Spondylolisthesis: Isthmic spondylolisthesis is another term for what you are describing. Basically one of the vertebrae is shifted foreword over the other. What has caused this condition in your case is a "defect" if the pars interarticularis, which usually stabilizes the spine. This condition can be associated with back and leg pain and difficulty standing or walking for long periods. Many treaent options.