A Private Blog Network (PBN) can bea site or a group of sites (domains) used to rank your site higher in search engines. The domains used for this technique are sites that used to be real sites before, but were either sold or left to expire.
PBN’s will be effective as long as the long links are a major ranking factor in Google. And according to ahref’s research the number and authority of pages and domains are the strongest predictor of ranking highly.
However, that doesn’t mean you can just buy any expired domain and link it to your website and expect it to give you a boost as it will also have a negative effect on your site.
Google has several filter’s in place to prevent SEO’s from abusing this method. So, it’s important that you know what you're doing before you try this yourself. As we know that Google wants backlinks that should be acquired naturally and PBN is by no means natural. It is actually considered as a Black Hat technique. Google is already against it and already banned a lot of PBNs.