Wat is Likelihood-ratio test?

The likelihood-ratio test of a null that imposes $r$ restrictions on $\theta$ rejects for large $\mathrm{LR} = 2\bigl(\ell(\hat\theta) - \ell(\hat\theta_0)\bigr)$, where $\ell$ is the log-likelihood, $\hat\theta$ its maximiser and $\hat\theta_0$ its maximiser under the restrictions.