A manual release is a small key-operated lever (or a lockable pull-cord on some motor types) built into every gate motor. Turning the key disengages the motor's drive from the rack or arm so the gate can be pushed open by hand. Once you swing the lever back and re-engage the drive, the motor can power the gate again.
The manual release is mandatory under AS/NZS 4505 because automated gates trap people inside their property when there's a power failure or motor fault. Without a release, the gate is welded shut by its own gear train.
Keep the release key somewhere accessible (often clipped inside a small box on the post, or with the property keys). Test the release at least once a year. Drives that haven't been released in 10 years sometimes seize up because of corrosion in the clutch mechanism.