Old stock Americans are people whose families have been living in the United States for several generations, sometimes all the way back to the founding of the country. Imagine if you had a big, old tree in your backyard that your grandparents, great-grandparents, and even great-great-grandparents all played under when they were kids. That tree would be like the United States for old stock Americans!
Just like a big family tree, some people can trace their heritage back to the first immigrants who came to America, while others are descended from people who were already here before Europeans arrived. These people have a special connection to the country and its history, because their ancestors helped shape it.
Being an old stock American doesn't mean someone is better or more important than anyone else. It just means that their family has a long history in the country. People of all backgrounds and histories are valuable and important in their own unique ways.