The locking options determine whether or not users can remove their own autobill plan.
Go to settings --> autobill to see the autobill plans on the project and create new ones.
Click "Edit" beside a plan to see the advanced options. 
The locking options are
- Lock user into plan after selected
- Lock user into plan after first payment
- Not lock the plan at all (turn both OFF)
The locks can be turned ON/OFF in settings --> autobill --> click edit by the plan.
Note: Plans can always be removed or replaced by the admin of the project, regardless of the locking settings.