Hi
@Variable is one of the @ functions in the Universe designer, it is used to call the value assigned to variables. This function is generally used in security implementation.
Please find the below link with more examples
https://www.youtube.com/watch?v=X9DJuV0_vY4
http://noreports.com/variable-and-variable-functions-in-business-objects/
http://wpselect.com/business-objects-universe-row-level-security/