Tinok shenishba means "a child taken captive."
Imagine you are playing outside in your neighborhood and suddenly, someone comes and kidnaps you. That means they take you away from home and your family without your permission.
The same thing happened to some Jewish children a long time ago. They were taken from their families and raised by people who didn't believe in Jewish teachings. As a result, they grew up not knowing much about their religion and Jewish way of life.
But when these people learned about their true background, they were called tinok shenishba. Even though they weren't raised as practicing Jews, they were still considered Jewish and welcomed back into the community.
So, the phrase "tinok shenishba" refers to children who were taken away from their Jewish families and raised elsewhere, but who later discovered their true identity and returned to their roots.