What's the meaning of TG for PCB?

Tg means the Glass transition temperature: the temperature at which the material stops acting like a rigid material and begins to behave like a plastic / softer.

Requirements are primarily driven by peak temperature, which is driven by component and PCB board size / thickness / complexity. With small components and a small board, Tg between 130C and 150C can be sufficient. Larger components and a larger board tend to need Tg closer to 170C to 180C.