It's pretty much what it sounds like. In their society they have police to monitor all your actions; they also have special "police" to monitor your thoughts. This is possible because you can commit "thought crime" in their society. Their task is to go beyond hunting down criminal acts and instead hunt out criminal thoughts, which are any thoughts that go against those approved by the state. For example, it is a crime to stand up and shout, "Down with Big Brother," and in their society, it is also a crime to even think it.
This view that thought can be a crime is shared by many totalitarian societies; Stalin is thought to have had 20 million of his own people killed, many of them on suspicion of thoughts against the state. It's pretty easy to control people if they know their neighbor disappeared last night for what he/she thought.
Parsons is a good example in the book (although he is almost comic). He is turned in by his children for thought crime and thanks them for it because, as he says, he wasn't even aware that he was committing thought crime.
the thought police is police that like they said monitors thought crime which is any thoughts against big brother/ the law. they used children to gather information from households and arrest people who have been caught committing thought crime.
The Thought Police are the people who who keep a constant watch on whether thoughtcrime is being committed. All suspicions will go under further execution. The Thought Police extends its reach with chileren of the family. Mr Parson was captured surprisingly just because his children reported him.