It means that the accident which you had some time ago, hasn't completely healed , or if it has then has left a degenerative part of your lower spine. The retropulsed fragment is either a part of the arthritic change or indeed a part of the old fracture.......whichever, it is sticking into the wrong place which may cause problems for you in the future.
Depending upon your clinical symptoms someone will have to make a decision as to the need for surgically removing this "bone fragment."