– How elastic that ball of personal space is.
– A high stiffness means that the vertex reacts immediately to another vertex enters their space.
– In this example I used a very simple mesh, made of only five points. I rendered the spheres around every vertex so you can appreciate better the difference between a small and a great value of Stiffness.

– First example, see how the smaller the Stiffness the more the vertexes can get close each other:

– Another example:

Blender Soft Body Self Collision Stiffness