Short answers, pulled from the story.
A peregrinus was a free provincial subject of the Roman Empire who was not a Roman citizen, from 30 BC to AD 212. Peregrini made up 80-90% of the empire's inhabitants in the 1st and 2nd centuries AD. They were governed by the ius gentium rather than the full protections of Roman civil law.
There were just over 6 million Roman citizens recorded in the last surviving quinquennial census return, dated to AD 47. This represented about 9% of the total imperial population, estimated at around 70 million at that time.
Peregrini could be tortured during official interrogations and faced summary justice, including execution, at the governor's discretion. They could not legally marry Roman citizens, could not designate heirs under Roman law, and paid a direct annual poll tax from which Roman citizens were exempt.
Peregrini could gain citizenship by completing a 25-year term in the auxiliary regiments, by special imperial grant for merit or status, or through bribery of governors or high officials. Emperors also occasionally granted citizenship to entire cities or tribes, as Otho did to the Lingones civitas in Gaul in AD 69.
The historian Dio Cassius attributed Caracalla's constitutio Antoniniana to a fiscal motive: extending two indirect taxes on inheritances and the manumission of slaves, which applied only to Roman citizens, to the newly enfranchised population. Caracalla had also raised both levies from 5% to 10%.
Roman conquest substantially reduced peregrini land ownership. Taking together imperial estates, colonial grants, and land sold to Roman private owners, a province's peregrini may have lost more than half their land. Pliny the Younger recorded that in the reign of Nero, just six private landlords owned half of all land in Africa proconsularis.