This song took me a while to understand but I finnaly got it right. Its definately about having a relationship and making love with a girl...

Hello, I've waited here for you everlong (Means, he was waiting for the girl to come in his bed)
Tonight I throw myself into, out of the red, out of her head she sang (Everytime he says, she sang or anything relating to singing, its because they make sound because they love it (orgasm)
I'll let you get the rest, it's pretty easy after this
The chorus is awsome
And I wonder, when I sing along with you (When i'm having an orgasm with you at the same) the rest is easy
Happy listening !