When someone is born in a certain country, they are considered a citizen of that country. But sometimes, people are born in one country and then move to another country to live there. These people are called "foreign-born."
So, when we say "foreign-born Japanese," we are talking about people who were born in a different country, but their ancestors are from Japan or their parents are Japanese. They might have moved to Japan or were raised by Japanese parents outside of Japan.
These people may look different from many of the people in Japan because they might have different physical features or speak a different language. But even though they were not born in Japan, they still have a connection to Japan through their heritage or their family.