A Fremen housekeeper for the Atreides, Mapes is the first Fremen introduced to both Jessica and Paul and is therefore the first Fremen the reader gets to know, as well. Through her, we catch glimpses of the Fremen lifestyle before Paul and Jessica are fully immersed into it in Book 2.
For example, when Mapes gives a crysknife to Jessica, we learn that the weapons must draw blood once unsheathed. We also learn of the water debt when Paul saves her life. These interactions provide the reader with the first clues about the Fremen society, a society centered on the importance of water and military-grade violence.
Unfortunately, Mapes she doesn't give us all that many clues before she is killed by Dr. Yueh, just before the Harkonnen attack.