The Lock toggle switch in the Tools menu controls whether you or your users can add or move stuff on your Board
The Tools menu is on the right-hand side of your screen. Click/Touch it to open or close. Once you have locked your Board a message appears at the top of the screen to remind you the Board is locked.
If you are not the owner of the Board (i.e. you have been invited to the Board) you will need to ask the owner to unlock the Board.
You can also lock Notes and Groups by double-clicking to open the Edit Panel and clicking the Padlock icon on the left. The Padlock icon will change from open to closed to indicate its status.
Tip. If you want to lock a Group and all of its contents you must also click on the Items tab in the Edit Panel and choose locked for the Locking items? option.