The Lock action prevents any changes from being made to an instance until it is unlocked. Locking and unlocking operations complete immediately. A locked instance will display a different lock icon in the instance details view, as has a smaller Actions menu:

Locking an instance is intended to protect it from accidental deletion or other permanent changes. It is not a robust security feature, as any authorized user of an allocation can simply unlock a locked instance and then do whatever they want with it.