The RPO is the Royal Philharmonic Orchestra, a well-known orchestra in Britain. An orchestra is a large group of players who perform classical music together — strings, wind, brass, drums — usually with a conductor in front. Philharmonic is a grand old word for a music-loving orchestra of this kind. The RPO is based in London and plays concerts there and on tour, and you will also see the name on recordings. When RPO is on a concert poster or a CD cover in this sense, it is this group of musicians, not a government office. People say the three letters, or they say the full name.