Radiant barriers (also known as reflective insulation) are a type of thermal (heat) insulations that inhibits heat transfer by thermal radiation (or heat by non-contact, i.e. across air).

A radiant barrier is a highly reflective material which significantly reduces the transfer of radiant heat either by reflecting it away from its surface OR emitting a very little amount of heat from the surface opposite the heat source offering a permanent way to control radiant heat. 

With a radiant barrier, radiant heat is BLOCKED instead of absorbed in to conventional mass insulations like fiberglass or cellulose.

In addition, radiant barriers are unaffected by humidity and will continue to perform at a consistent level no matter how humid the seasonal climate, unlike conventional mass insulations, making them excellent at consistently controlling heat.