This feature is used to protect your files from being changed or removed by another users.

You can lock or unlock the file by clicking on the toolbar;

Or, going to Edit --> Lock/Unlock File from the top menu.

The icon is located next to the title of the locked file in the Workspace pane and at the title of this file tab in the Content pane.

Another users can open the locked files to read only, but they cannot edit, save, delete, move or rename.