After the Civil War three amendments were adopted. The 13th ended slavery. The 15th said that the right to vote could not be denied on account of race. The 14th was a long one. It contains a clause that said states can’t make or enforce laws that abridge the privileges and immunities of citizens of the U.S. That was later used by the Supreme Court (not the current one) to apply most of the Bill of Rights to the states.
The 14th also contained a definition of citizenship. It’s the first sentence. “All persons born or naturalized in the United States and subject to the jurisdiction thereof are citizens of the United States and of the State wherein they reside.”
President Andrew Johnson opposed the 14th Amendment. He said that if it were adopted, even Chinese could become citizens. Right he was. The 14th says if you are born here, you are one of us. Doesn’t matter where you came from. Doesn’t matter who your parents were. Doesn’t matter if you are Jewish or Catholic or Muslim or atheist. You are one of us.
It’s called “birthright citizenship.” Trump and many in the Republican Party have pledged to get rid of it.