According to Zecharia Sitchin, Nibiru is the (supposedly native) name of the "Twelfth Planet." (The expression "twelfth planet" tends to create some confusion, as we traditionally accepted the existence of only nine planets - counting Pluto as a planet. An extra planet should therefore be considered to be the tenth. In astrology and ancient astronomy, however, the Sun and the Moon, -as being part of one solar system, together with the 'other' planets- were also called "planets.")

Many people, including quite some scientists, accept the existence of an extra planet (planet X) with an orbit around the Sun. These days, that planet is referred to as 'Planet Nine' (after Pluto was demoted).

Sumerian mythology tells the story of Marduk, or Nibiru, a foreign celestial body that entered our solar system. One of its satellites collided with Tiamat, a former planet in between Mars and Jupiter. Part of Tiamat became Earth, the shattered part became the Asteroid Belt. (See Tiamat or Marduk for more information).

Z. Sitchin speculated that Nibiru has a 3,600 year orbit around the Sun. Its inhabitants are supposed to be the earliest alien civilization to have walked the Earth. Sitchin refers to them as the Nefilim. These Nefilim would be (co-)responsible for the creation of Mankind, and for the creation of some of the oldest human civilizations, like e.g. the Sumerian. [Note: Earth's Galactic History: Nefilim could also physically refer to the "Lyran giants," who also played a part in the creation of mankind !!!]

There are plenty of texts and videos claiming Nibiru is approaching again, but those are absolute nonsense, as it would then be visible in the Southern hemisphere. And for what it is worth, Sitchin too did not expect a return of Nibiru for at least another thousand years.

Alex Collier claims the Nibiruans, or Anunnaki, came into existence as a new 'tribe,' hundreds of thousands of years ago, when a civilisation from Sirius B and one from the Orion system merged after the marriage of the queen of the Orion group to the king of the Sirian group.

It is interesting to know that e.g. Barbara Hand Clow describes lifetimes on planet Nibiru in her book "the Signet of Atlantis."

Note: some channelers claim Nibiru has an orbit that takes it around both our sun, and one of the suns in the Sirius system. The inhabitants of Nibiru would then be a Sirian species... Astronomically speaking, this seems highly unlikely.

Additional observations:

the ancient texts that mention Nibiru call it the abode of Marduk. The traditional interpretation is that where it is mentioned as a celestial body, it refers to a known celestial body, like Jupiter, or Thuban (which was the North star in Sumerian times), or Kochab (Ursa Minor).

Whereas Sitchin was in all likelihood right that the gods in ancient Sumerian, Babylonian, and Akkadian texts were extraterrestrial visitors, his interpretation of Nibiru as an extra planet with a 3600 year orbit is the most speculative aspect of his theories, and the one for which there is the least amount of evidence.