I think this song is about being absorbed in the feeling of loving and/or liking someone. It captures the kind of situation where you’re with someone and there’s that certain kind of energy that’s shared between the two of you and you’re not entirely sure when, where and how to act on it, but you know you must, because the person (and the feeling) consumes you whole.
The first part of the song is our speaker’s sudden realisation that they head over heels in love:
Down, down in my bones/Somewhere I'd never ever known/Right at the back of my head...
'Cause you talk to me/And it comes off the wall/You talk to me/And it goes over my head
So let's go to bed/Before you say something real/Let's go to bed before/You say how you feel
Here the singer captures the feeling of talking to the person he might love and everything she says flies right by him because he’s so desperately in love with her. Here, she might be discussing the details of their relationship, hence the next line “Before you say something real/how you feel”.
In the chorus he is of course talking about how he’s always known she was the one for him.
I try my best to unwind/Nothing on my mind but you
Oblivious to all that I'll owe/I'm hanging on/To what I don't know
Here, Justin explains how he can’t get this girl out of his head. He has tried relaxing (presumably away from her) and still she inhabits the craters of his mind. In the next line, I believe he’s referring to the girl’s former hesitation regarding their relationship. There might be some truth or accuracy to what the girl has been saying and he acknowledges it but still, decidedly, sets this hesitation aside because, this girl is the one he’s in love with.