This passage is part of the solution of an exercise in Differential Geometry. Let $S$ be a surface, and let $X: U \longrightarrow S$ be an orthogonal parametrization. If $N^X = \frac{X_u \wedge X_v}{|X_u \wedge X_v|}$, then $$ \langle X_{uu}, N^X \rangle \langle X_{vv}, N^X \rangle = \langle X_{uu}, X_{vv} \rangle - \langle X_{uu}^T, X_{vv}^T\rangle, \qquad \qquad \qquad (*) $$ where $T$ denotes the "part tangent to the surface", according to the book.

The exercise asks the reader to compute the Gaussian curvature for a surface parametrized by an orthogonal parametrization.

My questions are the following:

What does "part tangent to the surface" mean? What is the image of the second partial derivative, say $X_{uu}$? How to prove $(*)$?

Thanks in advance and kind regards