An I Win Card is, quite simply, a card that grants a player an automatic victory. This may refer exclusively to cards that grant instantaneous victory, to cards with effects so ridiculously powerful as to create an inescapable victory for all intents and purposes, or also to cards that cause a player to instantly lose. I Win Cards of are not encouraged, as they bring the game to an unsatisfying close prematurely (unless their purpose is to stop a game that has been going on way too long, whether for sheer volume of cards or modification of the rules to the point of an infinite game or similar insanity).
Note that cards that grant ridiculous amounts of points are not actually I Win cards, because they can often be countered by changing the win conditions. For instance, you could change the rules so that the player with the fewest points wins, thus negating the effectiveness of that card. Some games do not allow mega-point cards.