Short answers, pulled from the story.
A peregrinus was a free provincial subject of the Roman Empire who was not a Roman citizen. The Latin word means foreigner or one from abroad and derives from the adverb peregre which combines per meaning through and ager meaning field or country.
The constitutio Antoniniana issued by Emperor Caracalla granted Roman citizenship to all free subjects in AD 212. This decree excluded dediticii people subject to Rome surrender war freed slaves and applied during the reign of Emperor Caracalla ruling 211-117.
There were just over 6 million Roman citizens in AD 47 representing only 9% of a total imperial population generally estimated at 70 million at that time. Estimates place peregrini as the vast majority of inhabitants between 80% and 90% of all people under Roman rule.
Peregrini were excluded from service in the legions entirely because they could only enlist in less prestigious auxiliary regiments instead. At the end of a 25-year term both the soldier and his children received citizenship creating a stark divide between those who served in elite units and those restricted to supporting roles.
Peregrini were subject to direct taxes called tributum including an annual poll tax known as tributum capitis which became a major source of imperial revenue. Roman citizens were exempt from this specific levy while land taxes called tributum soli formed the most important revenue stream payable on most provincial land.