Jinchūriki (人柱力, literally meaning: Power of Human Sacrifice) are humans that have tailed beasts sealed within them. They exhibit extraordinary powers due to the immense chakra reserves they possess.
Background[]
During the ancient times, Kaguya Ōtsutsuki turned into the mythological Ten-Tails and faced off against her sons, which turned out to be her demise, however Hagoromo sealed the Ten-Tails within him and before his untimely death, he used his Rinnegan's "Creation of All Things" ability to separate the Ten-Tails chakra into 9 living bodies creating the Tailed-Beasts which can be sealed, but Naruto faced it upon himself to outlaw them and make him the Jinchuriki off every Tailed-Beast, thus making him the strongest shinobi alive besides the Ōtsutsuki's and Jigen.