If you have a small team or enterprise subscription, multiple team members can work on the same project (if access to the project is provided). When a user opens a topic for editing, the topic will be assigned to them and locked. When another user then opens the project, the topic that is already open will appear locked, with the name of the person who is currently editing it shown and cannot be opened until the first user has closed it.
Administrators will also have the option to overwrite a topic lock. When a topic has unlocked the user who has it open will be forced to close the editor. To unlock a topic right-click on the topic in the Project Structure screen and select Unlock this item