When a government criminalizes something, it passes a law that says that thing is now a crime. After that, people who do it can be arrested and taken to court. The thing being criminalized is usually an action: selling a drug, carrying a weapon, saying certain words in public. People also use the word about a group, in a looser, more political way: if you say a policy criminalizes the poor, you mean it treats poor people as if being poor were itself a crime. That is not the strict legal sense, though actually news and protest writing use it that way all the time. The word is formal. You meet it in law, newspapers, and arguments about whether a law is fair. The opposite move is to decriminalize something: the law stops treating it as a crime, even if many people still dislike it.