I think you are referring to the translation of Greek writings into Arabic by Muslims scholars from 750-1250 CE. Libraries containing these ancient texts fell under Muslim rule when Arab and Berber armies swept across the Middle East, North Africa, and Spain in the 600s-700s. Many Islamic cities became extremely large and prosperous during this era (such as Baghdad and Cordoba, both will more than half a million residents), and they supported large numbers of Islamic scholars. Certain Islamic schools of thought rejected the influence of Greek writings as pagan corruption, but many other schools of thought embraced Greek learning, particularly in science, mathematics, and medicine. Over the centuries, many of these texts that had been lost in Europe were translated into Arabic (and after the 1200s found their way in Arabic translation back to Europe, where they became a major inspiration for Renaissance thinkers).