Could someone more knowledgeable explain the concrete footing and foundation log barrier? I can't make out what it is between them...it looks like a burlap sack

Could someone more knowledgeable explain the concrete footing and foundation log barrier? I can't make out what it is between them...it looks like a burlap sack, but that can't be right. What is preventing the concrete from transferring water into the foundation log? There is obviously some material there, I just can't fathom what it is.