Energy

Energy is a non-physical substance within humans that allows them to sustain physical and mental activity.

First Anime
Jadeite was first assigned to collect energy for the Dark Kingdom. He usually looked for places where he believed he could collect energy from a large quantity of people. However, once the youma assigned to the task was defeated, usually the energy collected would vanish from his possession. After his death, Nephrite was assigned to collect energy. However, unlike Jadeite, Nephrite tried to collect energy from a single human, after sending a youma to raise the human's energy by causing their interests to become obsessions, and once their energy reached its peak, the youma would take form and absorb the human of energy. Like his predecessor, once Nephrite's youma were destroyed, the energy they stole would be restored to the victim. Queen Beryl used the energy collected to awaken (though not completely revive) Metaria. After her awakening, the Dark Kingdom shifted its goals from collecting energy to retrieving the Silver Crystal.

Shortly after the Dark Kingdom was defeated, two aliens, Ail and An arrived on earth, bringing the Makaiju with them. In order to sustain the tree's life (and therefore their own), the aliens sent Cardians to collect energy.

Trivia

 * In the original Japanese version of the anime, "energy" is pronounced enerugi like the German word "energie" as opposed to enaji, like the English word "energy".