EGR - exhaust gas recirculation. After the engine is up to operating temperature, the computer starts to apply vacuum to the valve (by actuating the solenoid on its top) which opens a passage from the exhaust manifold back into the intake manifold.

Blending a measured amount of exhaust gas with the intake air reduces combustion temps inside the cylinders, which in turn reduces Nitrogen Oxide compounds in the exhaust. Those compounds (called NOx for short) are a large component of smog and similar air pollution.

 

Lol, yeah looks like somebody doesn't allow jokes on the interwebs. The question was answered in the first couple of replies.

 

Now I feel so bad! Look at all the problems I caused!

RSchnier has it exactly right.

Then again, so does SoonerTruck