When something is happening "it's happening". This can mean a lot of different things. To put it into perspective it is usually something that is positive. On "My name is Earl" Joy tells of Earl's weird habit of waving his arms in the air shouting "it's happening" during an orgasm. Furthermore, before I ever saw that episode I was traveling home with my friend Rachel from a camping trip and I warned her blind a** of a bird in the road about 20 feet ahead. She said "it's happening" and sure enough feathers flew everywhere.
And spiritually speaking: Breathe, Believe and Receive. It's all happening.