This is a disc brake lock, which is an anti-theft device used for motorcycles, scooters, and bicycles. It works by locking onto the brake disc to prevent the wheel from turning, making the vehicle difficult to move. 
It is typically made from hard steel or alloy to resist cutting and tampering. 
Many models come with two keys, providing a spare. 
It is designed to fit disc brakes with a specific pin diameter, such as 7mm or 8mm. 
The lock is often waterproof and resistant to high temperatures and dust.