PPP = Project Protected Profiles.
Wikitree has Projects where volunteers work on a defined set of criteria that group individuals. For example:
1776 Project or Descendants of Pocahontas.
(You can find more here: Projects)
Profiles in these projects are identified and developed. They get properly sourced, their biography gets developed and their vitals are checked and corrected. Because we do not want these merged away or altered incorrectly, they get a special protection to prevent this. Thus Project Protected Profile.
Read more about them here: Project Protection
Thanks for a great question.