A U-lock is a type of bicycle lock shaped like the letter "U." It consists of a solid metal shackle that secures your bike to a fixed object, making it difficult for thieves to steal. U-locks are popular among cyclists because they offer a high level of security compared to other types of locks, such as cable locks. To use a U-lock effectively, it's important to lock both the frame and the wheels of your bike to a sturdy object, like a bike rack or a lamp post. This way, you can protect your investment and enjoy peace of mind while you’re away from your bike.