This metal frame inside the piano is known as the “plate.” Many people erroneously refer to it as the soundboard. The soundboard is in fact just that – a thin board behind the plate that resonates with the strings and amplifies the sound. The plate is purely structural.
And sorry, even though it’s often a nice shiny gold color, the plate is not made of gold! Or even brass, bronze or copper. Other than a few rare, experimental exceptions, all piano plates are made out of cast iron.
And that’s why pianos are so heavy.