Thermal Barriers are ceramic coatings that reduce the transference of heat. In the case of your
exhaust system, this results in lower temperatures under your hood and in turn lower temperatures of your incoming air charge.
Coating the top of the piston or combustion chamber of the cylinder head reduces the amount of combustion head that propagates through either part, keeping them cooler for the next fuel/air charge, this also helps to reduce the risk of detonation.
These are all explained in greater detail on their corresponding pages.