Crypto-Calvinism is a fancy way of saying that someone secretly believes in the teachings of John Calvin, a famous Christian preacher from a long time ago. Calvin had very specific ideas about how people should live their lives and how they should worship God.
When people talk about Crypto-Calvinism, they usually mean someone who is trying to hide their beliefs because they think other people might not like them or might think they're too strict or too hard to follow.
It's kind of like when a kid is secretly eating candy before dinner even though their parents told them not to. They know it's probably not a good idea, but they really want the candy and they don't want to get in trouble for it.
So, someone who is a Crypto-Calvinist might believe in things like predestination (which means that God has already decided who will go to heaven and who will go to hell) and that people should live very strict and moral lives. But they might not want to talk about these beliefs with other people because they're worried about being judged or rejected.