A Reather is a device used to contain a Mahri sprit when it is not in a human host.
In the Series Edit
The events of Season One are started off by Lyanna and Hunter stealing a reather in Episode One. It breaks in Episode Two, releasing the spirit into Ren. Later in the series, the Kah'Nath Master has a Reather which he intends to use to contain the Mahri spirit in Ren after extracting it by killing her.
Ren is clearly drawing a reather in Season One, episode one. She claims that she has no knowledge of it and that it is something that came into her head - it is unclear how this might have happened, as it occurs before she sees the device.