Card advantage is a way of referring to how profitable cards are when they are played, in terms of how many cards you have before, and after they are played. When discussing card advantage, ratios are usually brought up, with the greater number of cards you have either before or after the card is played determining if the card gains you advantage, and how much. Advantage is also broken down into types, depending on from where the advantage will take place (hand, field, etc.). For instance, "Pot of Greed" is considered a +1 hand advantage, as you're trading in one card (by playing the "Pot of Greed") for two (the cards you draw with its effect), thus gaining a net total of 1 card.
This type of rationing has become a minor point of debate among Duelists. Specifically, whether or not the card being played should itself be counted as part of the advantage ratio. Proponents argue yes, as it is still a card under your control, and once it has been played, you do not have access to it again, thus shortening the number of cards to take advantage of. Opponents, however, argue that you cannot gain the advantage of a card without playing it, and that whatever advantage you gain from it could make up for the played card's loss. Life Points do not factor into the argument, as more Duelists gain the mentality that they are expendable since a player who wins with as little as one Life Point still wins, and paying Life Points as a cost for a card is more acceptable than discarding a potentially life-saving card for that same activation. In such a case, holding onto certain cards will benefit the player in the long run if they get a monster and inflict more battle damage than they lost or cleared the field for attacks later would allow for a simplified state of play.
Counter Fairies are a good example of how card advantage works; as the Deck type's name implies, the Deck plays a large number of Counter Trap Cards while monster effects recycle the Counter Traps (and in the case of the "Heralds", help negate effects) and help gain Life Points, thereby denying options to your opponent and giving you both monster and Life Point advantage.