A lock is how Projector handles two people trying to change the same thing at the same time. When you open a project, you now own the lock. That means your changes will be saved on close. When someone else tries to edit the same project, they are presented with the following message.
This engagement is locked by [username]. Do you wish to steal the lock?
This second user has a choice. They can steal the lock from you, meaning you won't be able to save your changes. They can open the project read-only. Or they can cancel out and wait until later. If they choose to steal the lock then you will receive the following message when trying to save.
Unable to save engagement. Your lock has been stolen by [name].
In general, you should try not to steal locks from your co-workers as it can cause them aggravation.