What does is mean the norm of the differential on a tangent space?

Question

Let $M$ be a smooth manifold, $U \subset M$ open and $f : U \to M$ a $C^1$ diffeomorphism onto its image. Consider $(Df)_x : T_x \to T_{f(x)} M$. Consider a Riemann metric on $M$.

Can someone explain me with more details what does it mean the norm $$\| Df_x \|?$$

Thank you!