Pax Romana is Latin for 'Roman Peace'. From 27 BC to 180 AD, the Roman Empire enjoyed peace and prosperity, as well as good rulers, for the most part. Starting with reign of the first emperor, Augustus in 27 BC, the doors to the temple of Janus were closed for only the 3rd time in Rome's history. (The doors would only be closed in times of peace.)
Starting in 96 AD, the Roman empire was ruled by the Five Good Emperors, who were chosen for talent rather than family connections. This ended when the last good Emperor Marcus Aurelius had his son Commodus succeed him. In truth, for most of Marcus Aurelius' reign he was involved in warfare on the frontiers of the Empire, so some would say the Pax Romana ended when war with Parthia broke out in 162 AD.
The Pax Romana was not a constant time of peace, only that no major external wars existed. For instance all 3 Jewish revolts and the eventual diaspora of the Jews took place during the 'peace' of the Pax Romana.