What does it mean to say "nothing exists except through human consciousness?

If you think about it, how do we know that anything exists?  The only way I know anything is through my own consciousness.  I can not know anything except through my physical senses and my ability to think and imagine.

That is true for me, but also for every other human being on earth.

So you can argue at least that nothing would exist if there were nothing around to perceive it -- sort of like the old saying "if a tree falls in the forest and no one is around to hear it, does it make a sound?"

Or, as O'Brien says, the earth couldn't have existed before people since there would have been nothing to perceive it before people existed (since he would not believe in God).

I hope that helps.