Short answers, pulled from the story.
SPQR stands for Senatus Populusque Romanus, which translates literally as "The Roman Senate and People" or more freely as "The Senate and People of Rome". The -que suffix attached to Populus means "and" in Latin, joining the Senate and People into a single compound subject.
SPQR first appears in inscriptions from around 80 BC, during the Late Republic. Before this period, official Roman coinage simply bore the word ROMA rather than the senatorial formula.
SPQR last appeared on Roman coins during the reign of Constantine the Great, who ruled from 312 to 337 AD. Constantine was the first Roman emperor to support Christianity.
Commodus, who ruled from 180 to 192 AD, reversed the traditional word order on many inscriptions, writing Populus Senatusque instead of Senatus Populusque. This inversion was considered a deliberate provocation against the senatorial order, whose members he taxed to fund his donatives and mass entertainments.
SPQR remains in use as the official abbreviation for the comune of Rome in government documents and appears in the municipal coat of arms of the city. The medieval Roman Senate also struck coins bearing SPQR from 1414 until 1517.
Many cities adapted the formula by appending their own Latin initial to SPQ. Examples include Amsterdam (SPQA, Senatus Populusque Amstelodamensis), Dublin (SPQH, displayed on the City Hall built in 1769), Liverpool (SPQL on St George's Hall), and Chicago (Senatus Populusque Chicago on the George N. Leighton Cook County Criminal Courthouse). Naples struck coins with SPQN during Masaniello's revolt in 1647.