What is the meaning of Sample_mean?

Given a random sample ${\displaystyle \textstyle \mathbf {x} _{1},\ldots ,\mathbf {x} _{N}}$ from an ${\displaystyle \textstyle n}$-dimensional random variable ${\displaystyle \textstyle \mathbf {X} }$, a mean defined as

Source: wiktionary.org