I bought frozen pork ribs and the label said pre-frozen.  what does this mean, and is it safe to thaw and eat.  To me, it sounds like it was frozen twice. 
I've seen the term applied a lot to seafood. It means that the seafood was frozen when caught and for distribution to stores, then thawed at the store to be sold in its thawed state.   For ribs, I'd expect that the ribs were cooked then frozen at the plant. The store would then thaw the ribs for sale.  