This is regarding the concept of sample mean

I know that $\displaystyle \bar{x}$ is defined as follows

$\displaystyle \bar{x}=\frac{x_1+x_2+x_3+x_4+\dots+x_n}{ n}$

what I want to know is that what really is $\displaystyle x_i$

according to the notes and other references that i've referred, currently i believe that it is sample of

**values **that is taken from the random variable $\displaystyle X$.But if it is true how can $\displaystyle E(x_i)=\mu$ ? because $\displaystyle x_i$ is a constant.

Any help is greatly appreciated...