This is a great question. The Pax Romana refers to a time in Roman history, which lasted around two hundred years from the 31 BC, the battle of Actium, where Octavian defeated Antony, to the second century AD. Some scholars call this time Pax Augusta, in honor of the leadership and work of the first emperor of Rome, Augustus.
It is called a time of peace (pax means peace) for two reasons. First, the great civil wars of Rome were finished. In the late Republic Sulla and Marius fought, and Caesar and Pompey fought. The destruction was great in every way. With these civil wars finished, there was now peace.
It is also called a time of peace because there was a long stretch of prosperity. This is the time of great building projects like the Colosseum, and the great writers of Rome like Vergil, Ovid, Horace, and Tacitus. It was a time of great wealth. According to the great historian of Rome, Edward Gibbons, it was the Golden Age of Rome.