The Repreather is a piece of Equipment Lara finds in the Flooded Archives of Rise of the Tomb Raider. She takes it off a Trinity Flamethrower that falls into the water and on top of her after he has been skewered by the sword of a Deathless.
The Rebreather enables Lara to breath under water for an indefinite amount of time. When she dives she will automatically take out the Repreather and put it in her mouth. Without it Lara was in constant danger of drowning whenever she had to dive for a prolonged period of time.