A shared resource refers to a resource or data that can be accessed or modified by multiple https://en.wikipedia.org/wiki/Thread_(computing)[threads] or concurrent parts of a program. It could be any piece of data, object, file, database connection, or system resource that needs to be accessed or manipulated by multiple parts of a program at the same time.
Shared resources should not be used for https://en.wikipedia.org/wiki/Lock_(computer_science)[locking] because it increases the chance of https://en.wikipedia.org/wiki/Deadlock[deadlocks]. Any other thread could acquire (or attempt to acquire) the same lock while doing some operation, without knowing that the resource is meant to be used for locking purposes.
