its sound deadening.... there are other brands, brown bread, tsunami, e-dead etc... basicaly you have two options with sound deadening asphalt or butyl backed material, the asphalt is cheaper but smelly (at first) and sometimes one layer doesn't always do it. butyl is more $$ but no smell and usually one layer is all it takes. i highly recomend doing this to your doors at least, and if you are putting in a sub the trunk and rear wheel wells are always a good idea....(stops most rattling and makes the music sound better b/c there is a sound surface for the waves to bounce from not just some metal and wires)
