“Regular languages over a common alphabet are closed under union.”

Question

I just want to make sure I understand what this means. I am new to Formal Languages and this is my first week of courses.

Suppose we have two regular languages, $L_1$ and $L_2$, with a common alphabet.

The union of $L_1, L_2$ is $\{x:x∈L_1∨x∈L_2\}$

Does this mean that, for any string $s \in L_1$, we also have $s \in L_2$?