– The “spring” stiffness for Goal.
– The bigger this value, the more your mesh will be end near to its initial position.
– The smaller this value, the more your mesh will end near to its final position.
– You can see it as a spring joining the initial position (stiffness=1) to the final one (stiffness=0). Actually not, but helps to think so!

– In this examples see how lower values of Stiffness let the soft body go closer to the “final” position (that one for Stiffness = 0).

– Another example:

– In the next example you can see how 8 different values of Stiffness affect the simulation:

Blender Soft Body Goal Stiffness