The friction used by the Collider2D.
A Collider2D can be assigned a PhysicsMaterial2D either directly via to the Collider2D.sharedMaterial, indirectly via the attached Rigidbody2D.sharedMaterial, the global default or if no PhysicsMaterial2D is assigned then a default value is used.
Additional resources: PhysicsMaterial2D, PhysicsMaterial2D.friction