Wow. I had no idea. Thank goodness google is my friend.
A buck is $1.
"Ribs" refers to barbecued spareribs, where the meat is still attached to the bones and they are served still attached to each other in a rack.
A buck a bone means that you pay $1 for each bone in the rack of ribs.
(Have you ever eaten ribs? It might make sense if you have. I personally find it really messy.)