Without being too technical, the engine mounts are the four points of the engine that are connected to the chassis of the car.
If they were mounted solidly (as in race cars, which have solid metal mounts) all of the engine's rotational vibrations would be transferred to the body, resulting in a very uncomfortable ride.
The engine mounts for passenger vehicles are made of a compliant rubber compound (such as urethane, I'm being generic here), and act like shock absorbers to dampen the vibrations.
When they get old and brittle, they either stiffen up, or break. As they stiffen, the dampening properties are lost, resulting on more vibrations transferred to the vehicle. If one or more of them break, then the engine is free to move about the broken points, resulting in "clunky" sounds when changing gears, or tromping on the accelerator.
I don't know how they are mounted on your particular model, but the process usually involves loosening the motor mounts and raising the engine via a lift, allowing access to the motor mounts for replacement. For what it takes, it's best to have them all replaced, like you would with shocks brakes and tires.
It's not a job for the timid, and you run the risk of having an engine fall on top of you if it's not supported properly.
I've only replaced the mounts on my VW engine, but it was out of the car at the time anyway, so I am not sure of the absolute procedure for MBs.
Perhaps the more experienced mechanics on this forum can give you more insight as to whether the cost and expense you were quoted is valid...Good Luck!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car