This is a cable lock, commonly used to secure bicycles and other personal property.
Cable locks are generally made of braided steel wire covered in a plastic or rubber sheath, which makes them flexible and easy to use. 
They are typically used to secure a bicycle frame and wheels to a stationary object. 
While convenient, cable locks are considered less secure than U-locks or chain locks because they can be easily cut with bolt cutters. 
For higher security, it's recommended to use a cable lock as a secondary deterrent in conjunction with a more robust lock, such as a U-lock.