Nibiru (sometimes misspelled as "Niburu"), according to the pseudoscientific ideas of Zecharia Sitchin, is the name of a "twelfth planet" in the Solar System with a highly eccentric 3,600-year orbit that periodically comes close to the Earth, causing cataclysms. Sitchin based his claims on creative (mis)translations of ancient Sumerian texts and on no-less-creative interpretation of ancient Sumerian images.
The word Nibiru does in fact refer to a planet known to the ancient Sumerians. This planet is more commonly known as Jupiter, and is not expected to collide with the earth anytime soon.
 Planet recycling
After Nancy Lieder's Planet X fiasco in 2003, hers and Sitchin's imaginary planets became conflated and were peddled by various individuals as a cause for a 2012 apocalypse. It totally happened that time.