The document discusses tangent planes and normal lines to surfaces. It defines a tangent plane at a point P on a surface z=f(x,y) as having an equation involving the partial derivatives of f at P. A normal line to a curve at a point P is perpendicular to the tangent line at P, with slope given by the negative reciprocal of the tangent slope. The normal line to a surface z=f(x,y,z) at a point P passes through P with direction given by the gradient of f at P.