Tg, or Glass Transition Temperature, (Tg) is the point at which an epoxy changes state from a glass-like hard solid to a more flexible, rubbery material. It's an important measurement that helps you understand how an epoxy is going to perform within a service temperature range. The Tg also helps you understand other properties of an epoxy at room temperature, including shear, tension, and compression. Epoxies can still work above the Tg temperature, but become slightly more flexible.
Compared to Tg, HDT (Heat Deflection Temperature) is the temperature when an epoxy deforms under a load, which is commonly thought to be a more valid, functionally relevant number.