Short answers, pulled from the story.
The Constitutio Antoniniana was an edict issued in AD 212 by the Roman emperor Caracalla. It granted full Roman citizenship to all free men across the Roman Empire, and extended the same rights as Roman women, including the jus trium liberorum, to all free women in the empire.
According to the historian Cassius Dio, Caracalla's primary motive was to increase tax revenues, since non-citizens were exempt from many taxes. A second likely reason was to expand the pool of men eligible to serve as legionaries, since only full Roman citizens could serve in the legions.
The dediticii were excluded from the universal grant of citizenship. This category included peregrini dediticii, foreigners who had surrendered under treaty, and freedmen with a penal status who had been denied citizenship despite being manumitted from slavery.
The edict triggered a large-scale rewriting of private law, because citizenship carried a body of legal rules governing property, inheritance, and contract. Scholars describe this as a milestone in the provincialisation of Roman law, narrowing the gap between legal practice in the provinces and in Italy. It also established imperial constitutions as the primary source of Roman law.
Mary Beard draws a sharp distinction between the history of Rome before and after 212, describing the post-edict empire as "effectively a new state masquerading under an old name." She treats the edict as a turning point that fundamentally transformed the character of the Roman state.
Vast numbers of newly enrolled citizens adopted the nomen Aurelius in honour of their patron, Caracalla, whose full name was Marcus Aurelius Antoninus. Seven of the eleven emperors between Gallienus and Diocletian bore the name Marcus Aurelius as a result, including Claudius Gothicus, Probus, and Maximian.