What It Stands For
SPQR is an acronym for the Latin phrase, "Senatus Populusque Romanus," meaning "the Senate and People of Rome." The "Q" appears in the acronym because "populusque" is actually compound word made from the words "populus" (people) and "que," designating the word "and."
Meaning
"Senatus Populusque Romanus" designates the two primary governmental institutions of the Roman Republic (510 - 23 BCE). The "Senatus," or Senate, was a body of 300 Roman noblemen, and the "Populus Romanus" was the term for the "People's Assembly," or "Comitia Curiata." Thus the designation SPQR meant that an act or public work was approved by both houses of the Roman government, and was binding on or representative of all its citizens.
History
It is not known exactly when Rome adopted SPQR as its official seal. Although both the senate and the People's Assembly existed in the days of the Roman kings, the two acting together were not a sovereign body until after the expulsion of King Tarquinius Superbus and the establishment of the Roman Republic in 510 BCE. The acronym became so associated with the Roman state that it continued to be used even after the power of the two bodies was reduced to mere advisory status under the Caesars.
Ancient Usage
Roman officials frequently ordered the initials SPQR to be carved into public buildings; for instance, it appears on the Arch of Constantine, and at the base of Trajan's Column, both in Rome. The inscription was frequently included on Roman coinage. Perhaps most famously, it was engraved on the Eagle standards that Roman soldiers carried into battle.