There is a surface defined by setting implicit function g(x)=0, where x is a 3 by 1 column vector, denoting a point on the surface;
3X1 vector \nablag(x) is the Gradient(surface normal at point x;
3X3 matrix H(g(x)) = \nabla^2(g(x)) is the Hessian Matrix;
3X3X3 tensor...