Locking is in place for many objects or documents in the system. They can be viewed and where necessary even be removed.
In a live system removing a lock should not be a daily business matter, but it is good to know the tooling to view and remove locks is available. With transaction SM12 the locks that are in place are listed. Valuable information for a developer. If you need to know how some standard functionality locks it's business, so you can apply the same locks, ensuring a collision with sap functionality is not possible.
Are you locking yourself? Did you get thrown out of the system but locks are still in place? Is someone locking the customizing table you need to work on - and he's in a meeting? Good thing SM12 lets you remove lock entries. If you have the authorization of course.