I have two possible meanings to this song. But before I comment, I'd just like to say how amazing it is. Muse's cover is definitely the best, and now I can't listen to any of the others, they just sound too crap for words. Anyway, here goes:.
Possible Meaning no. 1) A depressed man has gone outside for the first time in ages. They take a look around and see just how beautiful everything is. When they realise this, they just feel so good that nothing else matters except this beauty. It feels like they've been born into a new life.
Possible Meaning no. 2) Somebody feels happy extremely happy for one reason or another, (mabye they're in love?) and they feel that everyone and everything needs to know, they can't let their feelings go unnoticed.