The Edit > Locking > Lock command locks the selected items to prevent them from being accidentally modified. Locked elements can still be selected and used for relative construction; Snap Guides are still available; and you can pick up their settings through parameter transfer.
See Parameter Transfer Between Objects.
Note: Locking a layer will have the same effect - it locks all elements which are located on that layer.
The Edit > Locking > Unlock command unlocks the selected item(s).
With the Unlock All command, you can unlock all locked elements, even if they are not selected. You can also use the relevant buttons of the Arrange Elements toolbar for these actions.