The Common Core State Standards Initiative is like a big list of things that teachers and schools want you to learn in each grade level. It's kind of like a map that helps teachers know what they should be teaching you and helps you know what you should be learning.
So, let's say you're in first grade. On the Common Core list, it says that you should be able to count to 120, add and subtract numbers up to 20, read and understand simple stories and write short sentences. Your teacher will use this list to create lessons and activities that will help you learn all of these things.
The idea behind the Common Core is to help all students in the United States learn the same things at the same time, no matter where they live or go to school. That way, if you move to a new town or state, you'll be learning the same things as other kids your age.
Some people don't like the Common Core because they think it's too strict or doesn't let teachers be creative. But others think it's a good idea because it helps make sure everyone is learning important things and prepares you for the future.