Something that's unlocked isn't latched or secured. You can open an unlocked door and walk right through it.
If your front door isn't locked, it's unlocked. Unless you lock it with a key or turn a bolt in the latch, it's going to stay unlocked. A more modern version of this adjective is used for mobile phones, and it means "able to operate on any carrier's network," rather than one specific network. So your unlocked smart phone will work with whichever phone company you choose.