Rewind gives Eri the ability to reverse a being's body back to a previous state.
When Rewind is activated, Eri's horn grows and glows brightly. Those that are in contact or in the vicinity of Eri at the moment of the activation start having their bodies rewound to previous physical states, undoing or bringing back injuries or modifications from the present or the past. She is capable of reverting people back to a point before they even existed, effectively erasing them from existence. Kai Chisaki even stated that this Quirk can rewind genetics, dubbing it a terrifying power.
During the climax of the Shie Hassaikai Raid, it is shown that Eri's strong emotions play a role in her Quirk's efficacy. Recalling how Mirio Togata risked everything to save her, a hope-infused Eri obtained the determination to help others, causing her Quirk spontaneously activate, and immediately defuse Kai who had overhauled himself with Shin Nemoto. And as Izuku Midoriya protected Eri, her Rewind remained active and continued to grow stronger, on which Izuku capitalized in order to activate One For All: Full Cowl - 100%. After the battle, Eri's Quirk began to run awry, had it not been for Eraser Head using his Erasure Quirk to neutralize her power, and eventually her horn shrank back to it's minuscule size.
Because of her young age and inexperience, as well as lack of information about Rewind, Eri is unable to properly control her ability, leading to the erasing of her father on one occasion.
Its nature as a Quirk that affects only living beings also restricts its potential for training due to the high risks involved. Because of this, Shota Aizawa is one of the few people that can stop Eri if she ends up losing control of her Quirk.
Rewind, as confirmed by Neito Monoma, requires the accumulation of a yet unknown element in order to be activated, otherwise, the Quirk will remain dormant, with Eri's horn remaining small and stubby.
Through several tests and experiments, the Shie Hassaikai was able to develop a drug that could target a person's Quirk Factor, rewinding it to a Quirkless state. The drug consists mostly of Eri's blood and cells, which contain the properties of her Quirk. The organization hoped to create an antidote capable of restoring a destroyed Quirk using similar methods, but Eri was rescued before they could do so.