definition: Let $A \subset R^n$. The Lebesgue outer meausre of A is $$m^*(A)=\inf \{ \sigma(S) \colon \text{S is a covering of A by closed intervals} \}$$
I know $\sigma(S)=\sum v(I_k)$, where $I_k$ is a closed interval
How am I supposed to understand this intuitively? And what does it mean by covering of A?