A time stone is an object with the capability of traveling throughout time and space. It was presumably created by Mallus.
At one point, Mallus presumably created time stones and later presented them to his most loyal followers. After Kuasa was resurrected, she was given a Time Stone, presumably by Mallus or one of his followers, and she used it to retreat after being defeated by Amaya Jiwe in Seattle 2042. Nora Darhk was shown to wield a Time Stone after the resurrection of her father, who at some point also acquired one. When Grodd was recruited, Damien Darhk gave him one as well.
Nora's time stone was destroyed by a bullet fired from a gun wielded by a younger version of her father when she and a kidnapped Ray Palmer traveled to 1962 East Berlin in an attempt to fix the Fire Totem with cold fusion, which a scientist from that era had invented.
Later, after Damien Darhk had died by becoming Mallus's vessel immediately before the demon assumed his true form, and Mallus himself had been killed, right before Nora was taken away to prison by the Time Bureau, Ray gave her Damien's time stone so that she'd be able to escape and start a new life, not wanting her to waste the second chance she'd been given by her father's sacrifice.
Later, with Constantine's life in danger after he'd used magic to save a boy's life, and Gideon was unable to help John recover, Ray concluded, that since magic had done this to John, magic was the only thing that could save him. He tracked down Nora, brought her to the Waverider, and convinced her to use her powers to save John. After John's life had been saved, Ray thanked her and told her to take the time stone and go back into hiding. Instead, Nora used the Stone to take them both to the Time Bureau. She subsequently gave the time stone to Ray and told him that she couldn't keep running; even though she knew that her father had sacrificed himself to give her a chance at a better life, she also knew that if she was always on the run, that life couldn't happen. Deciding that she could not be truly free without repentance, Nora squeezed Ray's hand one last time, leaving the stone with him, and allowed the Time Bureau's agents arrest her.
A short time after this, John Constantine used the time stone in the Legends' possession to travel back in time in an effort to change the fate of someone he cared about, although this ended up having catastrophic consequences.
DC's Legends of Tomorrow