Short answers, pulled from the story.
The tributum capitis was a poll tax that Rome imposed directly on individuals living in its provinces. It was collected first by private tax farmers called publicani and later by local magistrates and provincial senates.
Roman citizens were exempt from the tributum capitis, and it fell instead on provincial subjects. Towns that held the ius italicum, the privileges of Italy, were also excused from the tax regardless of their province.
No. Caracalla's 212 edict granted citizenship to nearly all free residents of the provinces, but it did not cancel the poll tax bill they already owed.
The poll tax was deeply resented across the empire and helped provoke the first Jewish revolt, led by the Zealots in Judaea in 66 AD. After Roman forces destroyed the temple in 70 AD, the emperor added a further poll tax on Jews across the empire, the fiscus judaicus, set at two denarii per person.
In 722, Emperor Leo III the Isaurian tried to introduce a poll tax in Byzantine Italy, sparking a revolt led by Pope Gregory II that set in motion the separation of Italy from the Byzantine empire. When King Aistulf of the Lombards later imposed his own poll tax of one gold solidus per head in 751, Pope Stephen II appealed to Pepin the Short of the Franks, an appeal that resulted in the Papal States being established in 756.
The early Christian writer Tertullian used that phrase to express how deeply provincials resented being singled out to pay a tax that Roman citizens did not owe.