What is an EGR valve what does it do?

it is a emission device exhaust gas recirculation , it take exhaust gasses and by control of the cars pcm or computer and a egr valve it introduces a calibrated amount of exhaust back into the engines air fuel mixture.exhaust being mostly a inert gas will lower combustion chamber temperatures for a more efficient burn that will lower nitrous oxide emissions. its also re-burns left over hydro carbons that do not get burned in the combustion process