It's about outing former communist spies. Poland was under communism for half a century. Communist security services were executors of the brutal oppresion of the regime. Those security services employed spies, who helped uphold the regime.
Communism fell in Poland 18 years ago. But people who were responsible for communist crimes, functionaries of the regime and collaborators (spies) were never judged or punished. Their names remained a secret. Some of them remained present in the public life (journalists, priests, politicians) and have been blackmailed and manipulated by the former security services that KNEW that this person had been a spy and does not want this fact to come to light.
Now, after 18 years, we are trying to come clean about this past, and to reveal the names and past deeds of those former communist spies. It's not about punishing them, but just letting the public know, that this or that public figure (the 'vetting' law applies to public officials, journalists, academics) was a secret agent of the communist regime and spied on his or her family, friends, colleagues.
The vetting law is about transparency, about coming clean about the past, about cutting the ties that still hamper public life in Poland.
Hope this helps.