Each rock layer tells of a different landscape or environment. The order of the layers tells the sequence of environments. In the Grand Canyon illustration below, the tidal flats of the Supai Group gave way to the river floodplains of the Hermit Shale. Later the area was a desert represented by the Coconino Sandstone. Then the area began to subside below sea level as evidenced the indicators in the Toroweap Formation. Finally, the Kaibab Limestone, which contains marine shell fossils, shows indications that the region had slipped entirely below sea level.
Don't worry that you might not have known each environment. If you could tell that the sequence of events from oldest to youngest is Supai Group deposition, Hermit Shale deposition, Coconino and Toroweap Deposition follwed by Kaibab Depostion, you are doing great.