PBN is Private Blog Network. They are mainly used to manipulate the search engine results and boost rankings of a page.
By it's very nature, they are against the rules with Google and Bing. You're building links that NOBODY will find, with the sole intention of ranking higher in search engines.
If you get caught on a PBN (meaning, if Google finds the PBN), you can get your site blacklisted from search results and receive penalties for your involvement.
It's not worth the risk. Build REAL content that people want to read and share it. You'll build links naturally during outreach campaigns and sharing.