What does the 'lock' checkbox on the 'Get Info' - 'General' - screen do?
This checkbox provides a means for the user to lock the item (i.e. a file or folder). Once locked, it can’t be changed or deleted.

Notes: 


When I say it can’t be deleted, what I actually mean is that if you try to delete it you’ll get a prompt alerting you to the fact it’s locked. However, this prompt does give you the option of moving it to the Trash anyway. 
Locking a file does not prevent someone from opening/viewing the file. In other words, locking does not password protect it.
To unlock an item, just go back into its Get Info screen and deselect the Locked checkbox.