It was first mentioned during Nico Robin's conversation with King Neptune, when she says there are three ancient weapons that bear the names of 'gods'.[1]
Like Pluton and Poseidon, Uranus is a dangerous weapon which can be used to rule the world or, in the wrong hands, destroy it. But as of yet, absolutely nothing else is known about it.
History
Uranus was a weapon known to have existed during the Void Century. Its actions during this point in history and what became of it since currently remains unknown.
Trivia
Like the other two weapons, it was named after a Greco-Roman deity. Uranus (Ουρανός) was the personification of the sky, the father of the twelve Titans, and the grandfather of the first six Olympian gods. "Ouranos" can literally translate to "Sky" or "Heaven" in English.
Uranus is, currently, the only Ancient Weapon whose abilities and location haven't been revealed.
References
↑One Piece Manga and Anime — Vol. 66Chapter 650 and Episode 570, Robin (Neptune in the anime) speaks of Uranus along with the other two weapons' names.