Ener Charging (エナチャージ Ena Chāji) is an action. A player may Ener Charge during their ener phase by putting a card from their hand or one SIGNI on the field into the ener zone. Additionally, some effects tell the player to [Ener Charge X], which is done by putting that many cards from the top of your deck into the ener zone.
Before WXK-P01 Klaxon, the term "Ener Charge" was limited to Life Burst abilities. However, many cards before WXK-P01 allow you to put cards from the top card of your deck into the ener zone, in a fashion exactly like Ener Charging.