Chorus:
The future's unwritten
words unspoken
Things could change iin just a day
Unwritten
I've still got time left to say
Those things I didn't yesterday
The song's kind of saying that even tough you've messed up, you haven't said somthing that you needed to say, or you're getting nowhere, anything could happen in the future and it isn't written. You've got time to turn things around, you can say what you want, and you can change things, so there's no point worrying about what's already happened and regreting things when you can decide what happens tomorrow.