A shared lock is a type of lock used in database management systems to allow multiple transactions to read a resource simultaneously. When a shared lock is applied to a resource, other transactions can also acquire shared locks on that same resource, enabling them to read the data without interference. However, no transaction can modify the resource while it is under a shared lock. This mechanism helps maintain data consistency and integrity during concurrent access. Once all shared locks are released, other transactions can then acquire exclusive locks, allowing them to modify the resource. Shared locks are essential for ensuring that multiple users can access data without causing conflicts.