Projection of the unit normal to the graph of function

Question

Let $u:\Omega \rightarrow \mathbb{R}$ where $\Omega$ is a subset of $\mathbb{R}^n$.

Define $\nu= \dfrac{Du}{\sqrt{1+|Du|^2}}$, where $Du$ is the gradient of $u$.

What $\nu$ rapresents? My book says that is the projection of unit normal to the graph, but the gradient in the expression of $\nu$ make me confused.Furthermore, what is the direction of unit normal to the graph in this context?