A surd is an irrational root left in exact form, such as $\sqrt{2}$ or $\sqrt{5}$. A square root like $\sqrt{36}$ is not a surd because it simplifies exactly to $6$.

When simplifying, look for the largest square number factor inside the root. You may split multiplication, so $\sqrt{ab}=\sqrt{a}\sqrt{b}$ for positive numbers, but you must not split addition, so $\sqrt{9+16}\ne \sqrt{9}+\sqrt{16}$.