It means the same thing. Even in English it sorta fits. I have one key. I have a key. "a" is simply an indeterminate article meaning it specifies an object as part of an arbitrary set vs a determinate article which "determines" the specific object out of a contextual set.
If I ask you to give me a key, then most likely I'm assuming you have a set of keys and it doesn't matter which I get.
Though, in practice if you really did mean ONE of something and it's not obvious you would stress it as I just did.