Hand advantage is the comparison of the number of cards in each player's hand.

A player with more cards in their hand should have a greater chance of countering what their opponent plays, due to the larger pool of cards at their disposal. (Of course, a player actually has to use the cards for them to be effective, and that's usually going to reduce his or her hand size.)

Playing cards from one's hand tends to use up hand advantage. The impact on hand advantage is calculated by subtracting the hand size before using a card from the size after using the card

Most cards negatively impact hand size values, often causing players' hand sizes to decrease as a Duel progresses.

Hand advantage is often off-set by the effect each card has on field advantage. For example, "Smashing Ground" is a "-1" for the user's hand advantage, but a -1 on the opponent's field advantage.

Some cards have varying effects on hand advantage, such as "Morphing Jar", which sends both players' hands to the Graveyard, and then each player draws 5 cards. Having an empty hand can make the effect of "Morphing Jar" a +5. Having a hand greater than five has the opposite effect. Some cards have variable impacts on field advantage, such as "Lightning Vortex", which is a "-2" for the user's hand advantage, but can be a "-1" to "-5" on the opponent's field advantage. It is generally best to use "Lightning Vortex" when it can destroy at least two of the opponent's cards, so the user's loss does not exceed the opponent's.

As you will almost always have a greater chance of beating your opponent if your hand advantage exceeds theirs, it is important to limit the number of cards you play per turn carefully to maintain a decent sized hand advantage.