'68 Dodge Dart GTS: Color carpet kit & recommendations

The mass backing is a rubber (or something like it) backing on the carpet. It provides more insulation & sound deadening. It also helps the carpet hold its molded shape better in my opinion. I additionally put a dynamat clone down in my car as well. I really like the results. Car is much quieter on the inside. I think the mass backing is worth the extra money, but that is just an opinion.