A "prom" is a tradition in high schools in the United States. It is a formal or semi-formal dance party, held towards the end of your senior year (final hear). The prom might be held at the school itself, in the gymnasium or another large space. Traditionally, each boy would ask a girl to be his "date" for the evening. So the "dream prom date" would be your ideal choice of boy or girl, either for that evening or perhaps to be your longterm boyfriend or girlfriend.