A "V-card " is not the type of card you would put in your wallet nor is it one you would use to play games with - well, you kinda could make a game out of it. Likewise, it's not something that will censor your life for you if you keep it in your pocket. Rather, it's somewhat ethereal and intangible.
In the simpler terms, a V-card is one's virginity. Thus, when one engages in sexual intercourse for the first time, rather than "losing one's virginity", one "trades in his/her v-card." I first heard this used by my girlfriend in some random conversation. According to her, in this terminology, instead of losing something by having sex and being stigmatized by a sense of loss, the idea of "trading in" implies a transformation, a conversion from one phase of life into another.
See also: V club, virginity