What is the button located down side of drivers seat on a Golf GTI for with symbol car lines going from roof downwards?

That's the button for the Electronic Stability Control (ESC) or Electronic Stability Program (ESP). The symbol you describe—lines representing a car sloping downwards—is a common graphic used to indicate this system. It allows you to temporarily deactivate or reduce the intervention of the stability control system, usually for situations like getting unstuck in snow or mud. However, it's generally recommended to leave it activated unless absolutely necessary.