OVERVIEW:
The MG‑114 “Berilly” is a Russian submarine-launched torpedo decoy designed to deploy a distracting acoustic signature underwater. It is an off-board, expendable decoy used to protect submarines by confusing incoming torpedoes.

DETAILS:The MG‑114 “Berilly” is “swimming, active and passive” decoy, meaning it can simulate submarine signatures to lure torpedoes away, Carried in external launch tubes on Russian nuclear submarines, including Akula- and Delta-series vessels.

FONCTION:
Launched underwater from a decoy tube.
Activates to emit acoustic noise mimicking the host submarine.
Guides incoming torpedoes toward itself, diverting them from the submarine.

SOURCE:
Wikipedia: Sonar decoy article