Star operation in CS theory
The $^*$ is an operator that takes a language (not only regular languages) $L$ and produces a new language $L^*$ called the Kleene closure of $L$. A language is a set of strings over a finite alphabet (alphabets are commonly denoted by $\Sigma$).

Definition of $L^*$

$$L^*=\bigcup_{i\in \mathbb{N}_0}L^i=L^0\cup L^1\cup L^2\dots$$

Where, $L^0=\{\epsilon\}$ and $L^n=LL^{n-1}$ for $n\gt0$

Example

Let $\Sigma=\{0,1\}$.

$\Sigma^0=\{\epsilon\}$

$\Sigma^1=\Sigma\Sigma^0=\{0,1\}$

$\Sigma^2=\Sigma\Sigma^1=\{00,01,10,11\}$

$\dots$

$\Sigma^*=\{\epsilon,0,1,00,01,10,11,\dots\}$

In other words, $\Sigma^*$ is the set of all finite-length binary strings.