One of the more confusing concepts for me as an English-speaker learning Spanish is that English has a clear difference between "to close" and "to lock"
The default words in Spanish abrir "to open" and cerrar "to close" also carry double meanings as "to unlock" and "to lock" respectively
In context that can make sense because if you're saying la tienda está cerrada it means "the store is closed" but could also mean "the store is locked"
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The default way to explicitly say "to lock (something; usually doors)" is to say cerrar con llave which means "to close with a key", and una puerta cerrada con llave means "a locked door" rather than simply a "closed door"
There exists a scenario where the door is closed but not locked, and that's when con llave can help
Another thing that I've seen is people will say con seguro or sin seguro which threw me off for a minute
See - el seguro is in my head "insurance" as in health insurance of car insurance; when it isn't seguro/a the adjective meaning "sure" or "certain"
But el seguro is another word for a "lock" or "deadbolt" [others include la cerradura or el cerrojo; and el cierre as well, though el cierre can also mean "fastener" for clothes like a button hole or a zipper]
So if you're trying to tell someone "the door's open" you can say la puerta está abierta "the door is open/unlocked" but if you specifically want to say "the door's open" as in someone is knocking and you're telling them the door's lock isn't in place you can say la puerta está sin seguro "the door's lock hasn't been put in place" like saying "I didn't use the deadbolt"
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Two slightly unrelated notes; if you're telling someone to "come in" as in giving them access, you can often say adelante which is literally "forward" - or you can use a command form of pasar "to pass"; saying pasa or pase [the usted command] when someone knocks is very common
The other unrelated note is that el cerrojo and la cerradura seem to be interchangeable; they're both related to cerrar ... and el cierre is often "the locking mechanism" on something
Two additional words you might need - la bocallave is "keyhole", literally "mouth-key", and el candado "padlock"
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A final and very tangential note - la llave has more uses than just "key"
It can refer to a "faucet" or "spigot" for water
It can mean "wrench", which la llave inglesa having thrown me off because that is the basic wrench you're thinking of but it's literally "English key"
And in wrestling, la llave is another word for a "hold" or a "lock", like headlocks or leglocks
I only mention it because there have been quite a few times where I could not figure out why llave was being used until I remembered it has so many secondary meanings
