Short answers, pulled from the story.
The Fourteenth Amendment was adopted on the 9th of July, 1868, as one of the Reconstruction Amendments following the Civil War. Its ratification required the formerly Confederate states to ratify it as a condition of regaining representation in Congress.
The Citizenship Clause directly overruled Dred Scott v. Sandford (1857), in which the Supreme Court had held that Americans descended from African slaves could not become citizens. The clause constitutionalized birthright citizenship for all persons born within the United States, with the exception of children of foreign diplomats.
The Fourteenth Amendment is the basis for landmark decisions including Brown v. Board of Education (1954, prohibiting racial segregation in public schools), Loving v. Virginia (1967, ending interracial marriage bans), Roe v. Wade (1973, later overturned in 2022), Bush v. Gore (2000, settling the presidential election), Obergefell v. Hodges (2015, extending marriage rights to same-sex couples), and Students for Fair Admissions v. Harvard (2023, prohibiting affirmative action in most college admissions).
Section 3 disqualified anyone who had previously taken an oath to the Constitution and then engaged in insurrection or rebellion from holding federal or state office. Congress passed the Amnesty Act in 1872 to lift most disqualifications, exempting all Confederates except former Senators, Representatives, federal judges, military officers, cabinet officials, and ambassadors. A broader amnesty in 1898 covered all persons then living who remained disqualified.
Before the Fourteenth Amendment, the Supreme Court held in Barron v. Baltimore (1833) that the Bill of Rights restrained only the federal government. The Fourteenth Amendment's Due Process Clause provided the vehicle through which the Court began applying Bill of Rights protections to state governments, starting with the Fifth Amendment's Takings Clause in Chicago, Burlington and Quincy Railroad Co. v. City of Chicago (1897). The Court has never incorporated the Third Amendment or the Fifth Amendment's Grand Jury Clause against the states.
New Jersey and Ohio rescinded their ratifications after Democratic parties retook those state legislatures, seeking to block the amendment's adoption. Congress rejected New Jersey's rescission as "scandalous" and declared in July 1868 that states cannot rescind a ratification once given. Both states formally re-ratified the Fourteenth Amendment in 2003.