Red Star

"The red star had hovered above the island of Mata Nui in the days when Kopaka and his allies first arrived. [...] The star was in the sky now above Spherus Magna, and had been since the arrival of the Mata Nui robot on the planet. With the robot destroyed, the red star would not be summoned into use again. Yet still it hung among the true stars, waiting, waiting for a call that would never come."

- Narration, The Powers That Be

The Red Star was an enigmatic celestial object that was located in the skies of Mata Nui and Voya Nui. It contained the mechanism that allowed the Great Spirit Robot to travel from planet to planet. The Star also served as a means of resurrecting the deceased beings in the Matoran Universe. Their bodies were teleported to the Red Star, where their bodies were either repaired or their consciousness were transplanted into a new body. However, this ability had limitations: The being needed to be within the Matoran Universe, and there had to be enough left of the deceased's mind and body to be salvaged; in other words, any beings who died on Mata Nui or left few solid remains could not be revived, and thus remained dead permanently.

The Red Star also served as a means for Matoran to determine the future - if the position of the Red Star in a constellation in the sky matched one of the prophecy carvings in the base of the Great Telescope, that prophecy was about to come to fruition.

History
The Red Star was created by the Great Beings after they had created Mata Nui to help the Great Spirit transport himself off planets. The Great Beings also placed the Kestora inside it.

The Star was also used to revive deceased inhabitants of the Matoran Universe, warping them to the inside to repair them if enough remained of the deceased and teleporting them back inside the Matoran Universe after repairing them. After teleporting Gaardus back to the Matoran Universe after reviving him, however, the Red Star's teleportation systems broke down; the deceased could still be teleported onto the Red Star but they could not leave.

On Mata Nui, the Red Star was used to predict the future. Lightning sent from the Red Star was responsible for the transformation of the Toa Inika. This lightning was part of a fail-safe, sent down should there be Matoran outside the Great Spirit Robot to create Toa if needed.

One of the tasks on the list of preparations for Mata Nui's awakening was to record the Red Star's location. While looking through the Great Telescope with her Akaku Nuva, Gali noticed life inside the star.

Bara Magna
When Tarduk was cleaning up the Arena Magna, he found a piece of metal where the only thing colored was the Red Star.

The Powers That Be
Right before his death, Tren Krom mentally screamed to Toa Nuva Pohatu and Kopaka an image of the Red Star. Shortly afterward, the two Toa spotted Gaardus in the trees and after a revelation about Gaardus's past, he agreed to take them to the Red Star. Inside Pohatu described it as "Like what the inside of a Toa might look like" and they encountered three Kestora.

Gaardus then teleported away, and the Kestora were flash-frozen by Kopaka. Kopaka and Pohatu then fled, and after some time heard strange noises. Kopaka used his Akaku Nuva to look ahead and saw something. The Toa then ran, and were encountered by the Onu-Matoran Mavrah, who offered them a place to hide. Kopaka and Pohatu followed him inside the room, where Mavrah introduced himself.

Trivia

 * The Red Star has revived many beings, but so far, it has never been said to revive any Rahi. The Red Star is incapable of reviving any beings made of Antidermis such as Rahkshi or Makuta.
 * The Red Star and it's resurrective ability is almost definitely a reference to Bionicle as a series of building toys, and the mutability and impermanence that entails.

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