Shields are a set of face-down cards that protect you against a direct attack.
At the start of the duel, both players place the top 5 cards of their deck face-down in front of them before drawing. When a shield is broken, or destroyed by an attack, it goes directly to your hand unless dictated by a card. When all of your Shields are broken, the opponent may attack you directly to win the Duel.
The shield zone (placed to the left of your deck) is where Shields are placed at the start of the duel. Some cards (such as Andromeda of the Citadel and Wild Sky Sword) allow you to place extra shields into your shield zone, while others, such as Baron Burnfingers, cause you to lose shields. Some creatures, such as Flare Inhibitor and Flamewing Phoenix, become shields when they are banished. There is no limit to the amount of shields in your shield zone. When all shields in this zone are broken, another attack is needed to win the duel.
When a shield is broken, it goes to your hand. If it has a lightning symbol and says "Shield Blast" next to it in its effect box, then you may activate its Shield Blast ability. In this case, you may activate a spell or summon a creature with this ability by paying no mana immediately after it goes from your Shield Zone to your hand, unless dictated by a card effect.