I'm reading about Sampling Distribution of a Sample Mean and don't understand what this variable x could be. Maybe some examples of what the variable x could be will help me understand.

The book states:

"Suppose that a variable $\displaystyle x$ of a population has mean $\displaystyle \mu$ and standard deviation $\displaystyle \sigma$."

Then it goes on to list some properties that do make sense to me.

Thank you. :-)