To me, this song is about a couple of kids who fell in love at the wrong time. It could have been magical, maybe even lasted forever but because of the flaws of being young- miscommunications, simple complications- they separated.
There was a big fall out and a lot left unsaid. After a significant span of time, one might have even started falling in love with another (twist of fate). This leaving the other to try and move on but knowing, whenever she is around him, there will always be that most challenging question "what if."
They can both feel the tension, actions are speaking louder than words. At this point, she realizes she can't spend her whole life wondering and this is the moment she put her whole heart, her whole soul on the line. Knowing the possibility of him feeling the same is minimal, she has to do it. This is the final moment where possibly years of suppressed hopes ad dreams will either be diminished with a final goodbye, or lovingly embraced with all eyes towards the future.