Pasted this from google...
Some vehicles have hydraulic, hydroelastic or hydro-mounts with hollow chambers filled with glycol or hydraulic fluid. Hydro-mounts act like a jelly-filled donut to absorb vibrations that would otherwise be transmitted to the chassis.
Hydro-mounts are often used with four-cylinder and V6 engines that don't idle as smoothly as a V8, and in luxury vehicles where motorists expect less noise and vibration. Some hydro-mounts even have internal valving and/or a solenoid to change the dampening characteristics at different rpms to better tune out unwanted vibrations. These are called switchable hydraulic mounts or electronic mounts.