Make the rigidbody "sleep".
Sleeping is an optimisation that is used to temporarily remove an object from physics simulation when it is at rest. This function makes the rigidbody sleep - it is sometimes desirable to enable this manually rather than allowing automatic sleeping with the sleepMode property.
Additional resources: Rigidbody2D.IsSleeping, Rigidbody2D.WakeUp, Rigidbody2D.sleepMode