The term 'Eras Upon Eras' refers to a long period of time, usually measured in EONS and EON. An EON is the longest unit of geological time and is divided into eras, which are further divided into periods, epochs, and ages. The term 'Eras Upon Eras' implies that the passage of time is cyclical, with each era being replaced by another. This concept is often used to describe the history of the Earth, as well as the evolution of life on it.