The main antagonists, the Dark Kingdom, were searching for the Rainbow Crystals to gain their power and ultimately conquer Earth. Throughout the series, the Sailor Scouts worked to find and protect the Crystals from the Dark Kingdom.
Each Sailor Scout was associated with a specific Rainbow Crystal and its corresponding planet:
* Sailor Moon: Silver Crystal (Moon)
* Sailor Mercury: Aqua Crystal (Mercury)
* Sailor Mars: Fire Crystal (Mars)
* Sailor Jupiter: Green Crystal (Jupiter)
* Sailor Venus: Gold Crystal (Venus)
* Sailor Uranus: Blue Crystal (Uranus)
* Sailor Neptune: Purple Crystal (Neptune)
* Sailor Pluto: Deep Crystal (Pluto)
Let me know if you have any other questions about Sailor Moon!