What does it mean when your lover says you're "the one"?

It depends on the one speaking the words! If you feel confident about what he/she tells you, then you are blessed. Some people can look you straight in the eyes and make your heart race with their words of love when it's nothing but a fart in the wind.
Hopefully that's not the case here!
The best of love to both of you!

Depends on who's saying it. I'd give it time. Don't rush into anything.... Enjoy the time you both get and make the best of it.
Too much thinking usually spoils the sparks.

Depends it it was preceded by the question "Who gave me herpes?"

It might mean that. Don’t count on it too soon though.

In that moment, probably.

One would hope... maybe

Yes!

You're gonna save the Matrix, Zion and the human race.

It means red flag.... Too many hallmark and lifetime movies

It should mean you are the one for the one that said it to you...

Maybe untill the feeling goes away after the first arugement

Depends on what he or she means by saying that.