SPQR comes from the Latin Senātus Populusque Rōmānus. This translates to The Senate and Population of Rome, the Roman Republic’s government.
Whilst the Roman Republic ceased in 27 BC, the subsequent Roman Empire took over the use of SPQR.
Use of the SPQR symbol was never abandoned. Throughout the long history of Rome, it can be found on public documents, monuments, coins and more. It is still used to this day as a symbol of Rome.