Onegaishimasu. This may seem an American, an eclectic, or even an outlandish zen answer, but IT is you. Once you understand that, you are already on the journey of your practice; the practice of your journey.
Babies seem to have IT. IT seems to be that natural, intrinsic force and energy imbibed and emanating from what some might call god or nature or higher self. As we develop our ego - that artificial construct handed to us by society to get around in the world - we identify with the false self created by that ego, and IT becomes distorted and its flow through us, as channels, becomes blocked off to varying degrees.
Over time those blocks become dis-eases. If, and as, we learn that this false-ego ego is not who were are, we're then able to begin the journey - the return - to what, to us, seems like a higher and more true state. We begin to awaken from our slumber. At that point, and various points and planes along the way, we experience and "witness" something greater taking place. We - by disconnecting the rigid hold of our egos - begin to allow IT to work and flow and create through us again.
That 2¢ from me, and 2¢ from IT.
In my experiences, neither of the definitions above are what "IT" is in relation to the "Internal Training" that has been the focus of attention in recent years. I would refer to Rob John's post if I wanted to read what "IT" meant. And if you wanted to understand further what "IT" comprises, buy Ellis Amdur's new book, Hidden in Plain Sight. Read through the new threads about Ellis' book. And read through some of the threads in the Non-Aikido Martial Forum, specifically those where Rob, Dan, and Mike are posting.