What is Frenet frame?

The Frenet frame at the current position of a point on a curved path is the pair of perpendicular unit vectors $(\vect{u_t}, \vect{u_n})$: $\vect{u_t}$ tangent, along the motion; $\vect{u_n}$ normal, toward the center of the circle of radius $R$ that best fits the curve there.