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It’s a value that can create the difference between Jell-O (endless jostling) and a rubber ball.
Basically a value that controls how long it takes for the object to return to its original shape.
In Blender they simply call it “speed” so I guess it’s the “speed” of the simulation to resolve forces.
This same variable also can be used to create the difference between silk blowing in the wind, and thick cotton blowing in the wind.