This lecture covers the TNB frame, consisting of the tangent, normal, and binormal vectors, which define a local rectangular coordinate system along a curve. It also explains the Frenet-Serret formulas, which describe the behavior of a curve using the tangent, normal, and binormal vectors. The lecture delves into the concept of the TNB frame moving along a curve, the formulas for calculating the TNB frame, and the implications of the TNB frame in understanding the local behavior of a curve.