Gauge" (ゲージ Gēji) is a resource used for playing cards and using abilities by paying their cost. The "gauge" can also refer to the zone that holds this resource. Cards in the gauge are always face down and cannot be looked at. Placing a card into the gauge is called "charging", and sending a card from the gauge to the drop zone at cost is called "paying".
At the start of the game, after both players draw their initial hand, they put the top two cards of their deck face down into the gauge without looking at them.
To increase the gauge, after the player draws a card to start their turn (even if it does not), they may choose a card from their hand, place it in the gauge face-down, and if they do, they draw a card. This is known as "Charge and Draw".
As Charge and Draw is fully optional, it is a way to remove unwanted cards from the player's hand and possibly replace them with more useful cards.
Currently there is no maximum limit of cards in the gauge.
Some cards have the ability to charge additional cards (i.e. Dragonic Charge) or forcefully remove cards from the gauge (i.e. Bandit Ninja, Goemon).
In the anime, the gauge is shown as a glowing card. When a card is placed in the gauge, it's depicted as a card flying towards the gauge that expands to match the size of what's already there. If there are multiple cards, the glow becomes yellow; the cards duplicate and glow white after duplication. From Season 2 onwards, the gauge card's backing can be seen. When a gauge cost is paid, the card(s) there turn(s) into a star and move(s) towards that card.