What is a "structure pool"

I am trying to figure out a simple structure datapack, using the pillager outpost as a base just because it was the simplest structure I could see. I tried to keep everything as close as possible, but I'm not really sure what to put for the "start_pool" or how to make a new one of those. The wiki seemed to assume I already had an understanding of that. I get a datapack validation failed when I try to open a world.