Another (an automotive industry favorite), the EGR stands for Exhaust Gas Recirculation, an emission control device that helps make the exhaust cleaner by recirculating some of the exhaust gases back into the engine for another burning.
All exhaust contains some small amount of unburned fuel. Even the most fuel efficient running engine has some- it is just a fact of the burning process. Older cars have much more unburned fuel in the exhaust than newer computer controlled cars and trucks.
By introducing a small amount of exhaust back into the engine, this helps burn more of the fuel.
This emissions device is the oldest and most widely used in the industry. An older vehicles the EGR valve is vacuum controlled. On most all of today’s vehicles it is electronically controlled by the ECM or PCM.