Truth-Seeking Balls (求道玉 Gudōdama, Viz. Truthseeker Orb) are black malleable orbs of high-density chakra possessed by those with the power of Six Paths Senjutsu.
In the manga, only Madara and Black Zetsu have ever used this name. For this article, I will shorten the name to "TSB" for brevity.
Truth Seeking Orbs are made up of chakra, but that's not what it is. It is actually Spacetime itself, or the absence of it.
One evidence of this is when 8 Gates Guy was moving so fast, it bends the Six Paths Staff on Madara's hand, which let Madara to immediately conclude that Guy was bending space itself.
You might think that this just means that this just means TSBs are subjected to the effects of gravity. That is not wrong, but that is also assuming Madara knows General Relativity, that gravity is the same as bending of Spacetime, which clearly isn't the case.
For Madara to know that he bends space just by looking at the bent staff, Madara must have prior knowledge that the staff must be intertwined with space itself.
The more straightforward answer comes from Black Zetsu that the Expansive Truth Seeking Orb is literally a new "Space".
This is quite similar to a Black Hole, an area absent of Spacetime, created from energy of high density.
Now you may think that the realms of General Relativity is probably too far-fetched for writers of a kids' show. But that's not the case.
When Kinshiki was taking measures for chakra, he measured in terms of space curvature. This is another example that the series have some form of understanding when it comes to General Relativity.
Abilities and Limitations
Truth Seeking Orbs have most if not all basic chakra natures within them, and through various combinations, it can achieve Kekkei Genkai, Kekkei Tota or beyond.
However, when one TSB has one chakra nature active, it will deactivate other chakra natures. For example, when Obito used one TSB for explosion, Tobirama can touch it without turning into dust.
This is because when the TSB is set for Explosion Release, it loses the attributes of Particle Release.
Another example is when Minato was in direct contact with Madara's TSB to teleport them away, there wasn't even holes on his cloak.
However, it was only until Obito became stable was he able to utilise Yin-Yang Release, the ultimate chakra nature that can nullify all other chakra natures.
However, because it is part of Senjutsu, it cannot nullify Senjutsu regardless of its chakra nature.
This is a recurring rule in Naruto that the same type of Jutsu cannot be used against each other.
Another example of this rule is when the light of Infinite Tsukuyomi can penetrate all things but the Susanoo of a Rinnegan user.
Since it's also a Six Paths Jutsu, it couldn't even harm those who possess Six Paths Chakra.
When Madara was trying to pulverize Might Guy into dust, Naruto could simply kick it away.
Users also lose control of TSB about 70 meters out. And as a result, it's very easy to lose them altogether.
Madara lost all of his TSB to the Kamui Dimension, while Naruto was only left 3 while losing most to Kaguya's Dimensions.
Naruto could also use it as the center of Tailed Beast Ball Rasenshuriken.
For Madara and Naruto, all of their TSB are behind their back by default. That is, with the exception of the staff held in Madara's hand.
For Obito, the two TSB that first appeared in his unstable state was on the back of his palm by default. Whenever it activates, the TSB will go through his palms.
After he stabilized, he gained 8 more TSB behind his back, while the two original remains at the back of his palms. The one in his right hand pokes through his palm and shaped into a staff.
"Big Ball" Truth-Seeking
When Juubito was unstable, he could make one TSB expand and contract suddenly, dissolving the ground it touched.
Not unlike that case, Ashura Otsutsuki was able to make avatar-sized Truth-Seeking Balls.
Expansive Truth Seeking Orb
Fueled by the chakra of all of Shinobi World, ETSB was able to create a super-sized TSB comprised of all chakra natures, in which the fourth databook described as "Chaos".
The databook also mentioned it can destroy the world. What "the world" refers to is up to interpretation. Some argue that it includes an Earth-Sun radius given that the other Kaguya dimensions contains Moons and illuminating stars.
Six Paths Shakujō
Original Article: Six Paths Explained (Naruto)
Six Paths Shakujō is the unofficial name for the Truth Seeking Staff possessed by the Ten Tails' Jinchuriki.
Hagoromo in particular possessed a dual-headed shakujo. One end has a ringed hoop, while the other end possesses a crescent shaped curve, representing the sun and moon respectively.
While Hagoromo has both ends of the staff, Madara and Obito have only the Yin end and Yang end respectively.
One peculiar exception is Madara holding the staff with Yang end when Obito extracted some Tailed Beasts Chakra from his body. The same thing did not reflect in the anime, which might mean it is an art mistake.
On very rare occasion, the Six Paths Sakujo can even turn into a normal TSB. When Madara was attacked by Night Guy, he was so injured, the Six Paths Sakujo that he used to guard himself with turned into a normal TSB, and never returned.
This might signal that Madara lost something in the process, which can likely be Six Paths Yin Power, in order to recover from his near-death experience.
Hamura Otsutsuki possesses his own Six Paths Sakujo, with only one end. However, his end is unlike either the Yang end nor the Yin end of Hagoromo's Sakujo.
There are no known reasons or explanation as to this. What we know is that at that point in time, he also possessed Six Paths Yin Power in the hand seal he used on Kaguya. Whether or not it is the same one as the one Hagoromo gave Sasuke is another mystery.
Six Paths Rods
While Naruto do not possess Six Paths Sakujo, he can turn his TSB into Six Paths Rods. Naruto demonstrated many use cases such as deflecting Madara's Inton Raiha, as well as sealing Limbo in place.
Madara also have his degraded Six Paths Sakujo as a Six Paths Rod which he can use as a projectile.
Six Paths Shield
By combining all ten TSB to form an omni-directional shield, one can defend himself from an explosion caused by multiple Juubi Dama.
Sword of Nunoboko
As mentioned in the manga and the databook,
The known users listed in the Fourth Databook are Naruto Uzumaki, Obito Uchiha and Madara Uchiha. Aside from them, Hagoromo Otsutsuki, Hamura Otsutsuki, Ashura Otsutsuki and Kaguya Otsutsuki are also known users in the manga.
Later on in Naruto: The Last movie, Toneri Otsutsuki also possessed TSB while in Tenseigan Chakra Mode.
The number of TSB also varies between users. In the original manga, it seems to correspond to the number of tails the user is Jinchuriki of. For example, Madara, Obito, and Hagoromo all have 10 TSB as Ten-Tails Jinchuriki, while Naruto only has 9 as Nine-Tails Jinchuriki.
Adult Hamura from The Last movie actually possess 9 Truth Seeking Orbs in an "S" shape.
What is weird about Toneri's TSB is that he has no fixed number.