(Apologies if this is a bit messy; I don't really know how to structure this)
This song has had me gobsmacked in the 19 hours since its release. I love the production, instrumentation, and the vocals from the girls; they truly do not disappoint musically.
Now, one of the more interesting things I've noticed is that a lot of its lyrical themes are directly translated into the music.
This is probably most apparent in the chorus with the sudden synth explosion mirroring "Oh my god, out of control". And the subtle tempo change after the "took away my metronome" line, which I think is really clever, as it can be interpreted as an accelerated heartbeat, or losing your footing in the presence of someone you love.
So by that, we can define the "Crescendo" as an outpour of emotions directed towards a lover, or an emotional surge reminiscent of love that causes one to lose emotional footing as defined by the sudden tempo change in the second verse ("took away my metronome").
Now for the interpretation, I think "Crescendo" tells a story of a relationship that's just starting out, and dealing with the sudden outburst (or crescendo) of emotions that comes with it.
I think this is most apparent with Bae's lines, with her expressing the uncertainty of the relationship, thinking that—just like a fairytale—it's just a fantasy, and there's no guarantee of a happy ending. That uncertainty might also refer to the "deep in shallow" line as well, which works really well if we're talking about a new relationship.
But later on, as the song ends, we get "Now I can feel Crescendo. Even if I give you everything," from Bae, which can be interpreted as coming to terms with the uncertainty, and accepting the overwhelming nature of those emotions—redefining the "crescendo" as some sort of "high" you feel whenever you're with someone you love. And that sensation, that outburst of love and emotion, fills you with the desire to give them everything.
Perhaps the song doesn't really even tell a direct story, and is more focused on capturing and conveying that outburst of emotions into a literal, musical, crescendo.
Thanks for reading!