It's the magnitude of the gradient of $F$.
$grad F$ or $\nabla F$ denotes the gradient of the field F and thus |grad F| is the magnitude of the gradient of F.
It's common practise, but just a matter of taste, to use $grad F$ over $\nabla F$ in order to guide the eye easier as to what is being done or used (similar to $curl A = \nabla \times A$).