Shūkyō nisei is a fancy word that means "second generation religion" in Japanese. It's a term used to describe the children of Japanese immigrants who were born and raised in a new country, usually the United States.
Imagine that your mom and dad moved to a new country before you were born. They had grown up in a different place with different customs and beliefs. But now they live in this new country and they still want to practice their old religion. So they go to a temple or a church where they can pray and worship, just like they did back home.
But because you were born in this new country, you might not feel the same way about that religion as your parents do. You might have different ideas and beliefs because you grew up in a different environment. So, you are part of the second generation of people who are practicing that particular religion in a new place.
That's what shūkyō nisei means: being the child of people who moved to a new country and continued to practice their old religion, while you grew up and became part of that religion too.