The Moon's history is divided into pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican periods from oldest to youngest. At its simplest, surfaces with more craters have been exposed to space for longer, so are older, than surfaces with fewer craters. Volcanoes can spew out large lava deposits that cover up old cratered surfaces, obliterating the cratering record and resetting the crater-age clock.
To show you how this calibration changes with time, here's a graphic developed from the previous version of The Geologic Time Scale , comparing the absolute ages of the beginning and end of the various periods of the Paleozoic era between and Relative ages are more accurate, among scientists anyway. Any craters or lava flows that happened inside the Imbrium basin or on top of Imbrium ejecta are younger than Imbrium. Earth is an unusual planet in that it doesn't have very many impact craters -- they've mostly been obliterated by active geology. The book design presents the material in easily digestible chapters, with clear delineations between sections and well-placed artwork. Igneous Rocks and Intrusive Igneous Activity.