Hi Jon thanks for posting. Are you referring to the lock icon in the top menu bar above your canvas? If so you will need to select an object (or multiple objects) on your canvas first. Please see the below GIF for a demonstration.

This feature allows you to lock style formatting or size/position changes to aspects of your diagram rather than the whole document. We do have a separate content locking feature that allows you to set your document status to locked and prevent any further changes- please see this section of our Manage Document Status article for more information. Please note that this is only available to users on Enterprise accounts. Feel free to let me know if you have any further questions.