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 their 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.
Various abilities, mostly on green and colorless cards, allow you to Ener Charge.
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 of your deck into the Ener Zone, in a fashion exactly like Ener Charging.