What is auto-rotation?
When a helicopter engine fails, the pilot lets the rotor blade rotate freely pushed by the wind as the helicopter is falling out of the sky. This is called auto-rotation.

Normal rotation is with the engine powering the rotor.

The most important goal of auto-rotation is to get enough lift right before touch down by flaring so the landing is not a crash.

You would also enter into autorotation when the tail rotor becomes ineffective and you can't recover it otherwise.  Once the tail rotor is back into action you can drive the rotor again.