Private Blog Network or Perosnal Blog Network refers to creating multiple blogs sites such that all are interlinked or pointing to one domain in some or the other way.
This works best for Black Hat SEO as individual sites of PBNs have already gained Seo backlinks, so when a new link for SEO is added to the PBNs, it’s SEO score shoots up. All in all, PBNs are dangerous for any site and one can be penalized for using PBNs as marketing strategy.