The Lock button enables you to freeze all elements on the page. When elements are locked you will be no longer able to use the mouse to move, resize or rotate elements.
An elements size, position and rotation can still be changed by opening the settings dialog box of each element in the Advanced tab, common to all elements.
The lock elements command is completely independent from the Enable mouse action option found under the Advanced tab of the Settings forms. In fact, when the Lock elements is active, it is still possible to double click on each element to enter the Settings dialog box (as long as the Enable mouse action was not disabled for that element).