It is an important measure of a deck's performance - take this example; Terror, is a good card. It wins games. However, a deck with 40 terror and 20 swamp isn't good, becuase it never wins. This is an extreme example, but it demonstrates that win conditions are important, and goldfishing primarily tests the viability of your win condition.
You can find the definition of this and many other Magic-related terms in the Magic Player's Dictionary, stickied at the top of Magic General. Please post any terminology-related questions there. Thread closed.
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"...because without beer, things do not seem to go as well."