Hi Statalist

In many of the questions posted, the following syntax has been used: e.g. $covariates or $xlist. Can I please ask 1) what exactly does the $ symbol do in a Stata command and 2) how does one go about creating something like $xlist?

I have tried - help $ - and googling search terms like "Stata what does $ mean" but haven't yet found the answers.

Thanks.