Birthright citizenship in the United States means that any baby born here is automatically a citizen of the United States, even if their parents are not citizens. This is based on the 14th Amendment, which was part of the United States Constitution. That means that anyone born in the United States is automatically a US citizen, and they don't have to do anything special to become a citizen. This is different from some other countries, where you have to do special things (like taking a citizenship test) in order to become a citizen.