Explanation for the regular expression for the set of all string over the alphabet {0,1}
It is a Klenee operator and it means all number of $(0 + 1)(0 + 1)\cdots$ with all possible length. As $(0+1)^{10}$ means $(0+1)(0+1)\cdots(0+1)$ for ten times. Hence, All possible combinations of $0$ and $1$ are comming in $(0+1)^*$.

In the other words by the definition, it means the union of all $(0 + 1)^i$ for all $i$ from $0$ to $\infty$.