It's everything not related to the real world or your waking moments.
It's quite simple, really. Just picture yourself living in a place and time that doesn't exist yet, or perhaps once existed. Picture the people, the surroundings, the vibe of the place. Then, thrust yourself into an adventure of your own, be it escaping from some thugs out to get you, to finding out that you're the lost heir to a long-forgotten throne, or to battling a villain who slaughtered your entire family. Simply put, let your imagination run free and flow in an endless stream.
The list could go on forever. But that's the beauty of imagination: anything is possible.
It's the best escapism one can ever find. It's all the fantasies we could never live out in real life, all our dreams come true and all our wishes granted, even if it's only from that small corner of our mind that plays them out right before our mind's eyes.
Basically, it's everything that life isn't.
It's our shelter from the real world when we find that we cannot face it. It takes us to a safe place, where we control everything and nothing can ever go wrong. It's consolation, comfort and support, not provided by someone else, but by the workings of our mind. It may be just a trick of the mind, but oh what a trick it is.
It's our sword and shield. Our army and fortress. Our aggressor and pacifier. Our battleground and safe haven. But moreover, our will, strength and support, to keep us going no matter what.
It's perhaps the best gift that we as humans have.
We cannot escape the realities of life. But we can seek sanctuary in our imagination.