PBN (or Personal Blog Network) is an SEO tactic used by webmasters to get high SERPs for their Blogs. This technique involves reviving high quality expired domains and adding new content with links to the main Blog (Money Site). This is a Black Hat SEO method and has been effective for many webmasters and Bloggers.
But Google and other search engines hate PBNs (for obvious reasons) and they try their best to penalize the websites using PBNs. This is the reason why most of the PBNs that you build, gets deindexed by Google and you end up losing thousands of dollars.