I wonder what is this filling the octree means?

I am now trying to render it on my brothers computer with 2 gigs of memory and double core cpu. It doesn't take so much memory here as it did on my computer. but it is still working on filling the octree. What that is I don't know and it has counted up to ca 2609000. And is that lot?
It have been working like that the last 4 hours.