Let $\Omega$ be an open bounded subset of $\mathbb{R}^n$ with $\partial \Omega$ being $C^2$. Suppose $u\in H^1_0(\Omega)$, $f\in L^2(\Omega)$ and $\nu>0$. It is said in the Navier-Stokes Equations by Constantin and Foias (in the chapter about weak solutions of the Stokes Equations) that
$\nu \Delta u+ f$ is a distribution in $H^{-1}(\Omega)$
where $H^{-1}(\Omega)$ is defined as the continuous dual of $H^1(\Omega)$ and $$ H_0^1(\Omega):=\overline{C_c^\infty(\Omega)}^{\|\cdot\|_{H^1(\Omega)}}. $$
I can understand that $L:=\nu \Delta u+ f$ is a distribution since $$ H_0^1(\Omega)\subset L^2(\Omega)\subset L_{\textrm{loc}}^1(\Omega) $$ and any function in $L_{\textrm{loc}}^1(\Omega)$ can be identified as a distribution, which is a function from $C_c^\infty(\Omega)$ to $\mathbb{R}.$
Here is my question:
What does "$L$ is in $H^{-1}(\Omega)$" mean?