Ab urbe condita is a way to measure time.
The phrase is Latin. It literally means from when the city was founded. The city talked about is the city of Rome. This way of measuring time was not used in Classical antiquity. The first to have used it was the historian Orosius, about 400 AD.
Marcus Terentius Varro found out that the city of Rome was founded 440 years after the fall of Troy. Romulus and Remus founded it. Varro thought that was in the spring of 753 BC.
Marcus Porcius Cato Censorius thought that Rome was founded one year after the 7th Olympic Games. That would have been in 752 BC.
Ab urbe condita is also the main work of Titus Livius, a Roman historian.