A regeneration alcove is a recess in which a Borg drone stands to rest, recharge and maintain its implants, replacing sleep for members of the Collective.
Borg vessels are lined with alcoves where drones connect to the ship to regenerate, standing upright and inactive. While regenerating, they remain linked to the Collective.
After Seven of Nine joined Voyager, the crew installed an alcove in one of the ship's cargo bays so she could continue to regenerate. Icheb and other liberated Borg children also used alcoves there, and the cargo bay became Seven's personal space aboard the ship.