A '1 For 1 Card', is a card that you effectively sacrifice to take away 1 single card on your opponent's side of the field, or to replace one that's either on the field, or in your hand. The following are examples of '1 for 1's':

Examples of one for one cards include "Confiscation", "Smashing Ground" and "Reinforcement of the Army".

This ties into the duelists' concept of "card advantage", and the long-standing argument behind it; think of advantage as a ratio, determined by benefits and costs - "benefits" is defined as the number of cards you gain back and/or the number of cards your opponent controls that you can get rid of, while "costs" is defined as the number of cards you have to give up in order to use a card's effect. The more the benefits outweigh the costs, the better the advantage, and vice-versa (for example, Pot of Greed would be considered a "+1 advantage", because you're gaining 2 cards at the cost of 1, thus giving you a supposed net gain of 1 card).

While a simple way of calculating how useful a card can be, it does have its flaws that are the sources of argument with duelists over it. First, this way of calculating "advantage" only considers card amounts, either in your hand and/or on the field, and completely ignores other costs, such as paying life points, reducing a monster's ATK or DEF, or skipping turn phases. Second, and the most important point being argued, is whether or not the card being activated itself should be counted for calculating advantage; those that argue that it should state that it is still a card that you have to give up to use its effect and thus should be counted, while those that argue it shouldn't say that the card's effect can't be activated - and thus, its benefits can't be reaped - without playing it, and thus should be excluded, instead having "advantage" only count any additional costs to the card's activation. As such, the entire concept of a "one for one card", and whether the above examples of such cards should truly be considered such, all depends on the particular duelist being asked.