What exactly is Pluton and regarding that, what is so special about the Void Century?
Pluton is one of the 3 Ancient Weapons capable of mass destruction. The other 2 are Poseidon (currently Shirahoshi) and the ancient ship, Pluton.
Uranus is not shown yet but if speculations are to be considered, it is said that it is a giant machine that can eradicate a single country.
Void Century as its name implies, a
century that is hidden by the World Government to the public. It is a century where the 20 kingdoms fought against the "D" and won. After the War, they've joined together and formed the World Government as their acting police and their descendants are the Celestial Dragons. The only exception was the Nefertari family who refused to join them in Mariejois, the holy land of the Celestial Dragons.
Not much is known what happened during the void century because it is clouded in mystery. Only a few people are known to have knowledge about it, including the Ohara people where Robin came from, and Roger himself.
It is heavily implied that the Void Century, D, and Poneglyphs are heavily connected.