Looking at the source code for the FChaosPhysicsMaterial class, I'd say Static Friction is specifically the resistance to the start of motion. It's the threshold of force required to transition an object from a stationary state to a moving one. This parameter is not a factor once the object is in motion; at that point, the regular Friction parameter takes precedence.
Let's say you're simulating a block of ice, so you might set a Static Friction of 0.1 to indicate that not much force is needed to start the block moving. The Friction might be set to 0.2, indicating that once moving, the block still slides easily but with slightly more resistance than when it was stationary.