This is what website owners go after when they just can't beat the competition. More juice to push the site up in the ranks. A private blog network (PBN) is an old domain that has high domain authority and trust flow. When you set these up, it takes research to find the right domain for this. Checking if the domain was marked or used for spam purposes is very important. You wouldn't want to get that mark too.
Once you buy one, you keep it private, write an aritcle about your main money site and add a link. That's about it in a nutshell.