It is the job of the Thought Police to uncover and punish thoughtcrime and thought-criminals. They use psychology and omnipresent surveillance (such as telescreens) to search, find, monitor and arrest members of society who could potentially challenge authority and status quo, even only by thought, hence the name Thought Police. They use terror and torture to achieve their ends.
It also had much to do with Orwell's own "power of facing unpleasant facts," as he called it, and his willingness to criticize prevailing ideas which brought him into conflict with others and their "smelly little orthodoxies."
In the first half of the twentieth century, prior to the publication of 1984, the Special Higher Police (Tokko) in Japan were sometimes known as the Thought Police.
The term "Thought Police," by extension, has come to refer to real or perceived enforcement of ideological correctness, or preemptive policing where a person is apprehended in anticipation of the possibility that they may commit a crime, in any modern or historical contexts.