$\frac{\partial \textbf u}{\partial t} = -(\textbf u \cdot \nabla)\textbf u + \textit v\nabla^2\textbf u + \textbf f$

What's $\nabla^2$ mean? I know it's divergent, but why is it divergent squared? Or is it divergent of u squared? But then why isn't the square after the u?