Plain English Meaning
Because the body of a high-wing aircraft hangs below its wings, it naturally swings back to level flight when tilted, much like a weight hanging beneath a pivot point.
Full Definition
A lateral stability characteristic in which a high-wing aircraft tends to roll back toward level flight because the fuselage and vertical tail hang below the wings, acting like the keel of a sailboat. When the aircraft is sideslipped or banked, the relative wind striking the side of the fuselage below the wing creates a pendulum-like restoring effect that returns the aircraft toward wings-level.