It tells you how many millimeters the plane of the mounting face of the wheel is deeper than the plane of the centerline of the wheel (positive offset on the German system). I used to know what German word the "ET" stood for, but I can't think of it right now.

Bigger numbers move the wheel closer to the suspension parts, so some of the similar looking wheels from later model Mercedes cars with ET 35 or more could lead to interference on the inside of the tire or wheel, depending on what size tire you put on, while rims with very small ET numbers (like some of the AMG special wheels from the '80s) may result in wide tires rubbing on the fender lips.

Deviating from standard (which would probably be ET 30 on your car) also affects the "feel of the road" in the steering one way or the other, but ET 21.5 would not be likely to do more than make the the car feel a little more responsive.