This means that the L5 vertebrae is slipping off the sacrum. The slippage is rated by percents. Grace 1 is 0-25%, Grade 2 is 26-50% and so on. So your slippage is somewhere between 26% and 50%.
I have this and am having a spinal fusion surgery to fix it on April 7th. Surgery may be indicated if it is causing a lot of pain and conservative treatments such as physical therapy have failed.
I would encourage you to do an internet search on your diagnosis. There is a ton of info out there on the subject that will explain it a little better than I can.
