The search engines determine your page rankings, but you can do things to up your rankings. What is a ranking? Well, it comes down to the position your website when someone searches for terms that are related to what you sell. You do not want to be on the second page of search results or even farther down. You want to be in the top five or so. You could buy a high ranking by paying for a search engine ad, but those can be expensive. Also, people recognize them as ads. Some people buy PBN links to up their page rankings for their business websites.
A PBN is a private blog network. There are thousands and thousands of expired domains that have been around for a long time. The search engines constantly scan the Internet. If they find reputable domains, such as blogs, that have links on them back to your site, then it makes your site look more reputable. That then gets you a higher page ranking.