Prudence represents the ability to make wise decisions, judge actions apprpriately at any given circumstance. She is shown with a mirror, standing for reflection (of thought) and knowledge; her two faces (reminding of XXX Theologia) hint at the ability to look into the past and the future, i.e. to weigh cause and effect. The little dragon at her feet might have been intended to be a serpent as the symbol of wisdom, or actually a dragon to symbolize Prudence's skill to judge the evil (and thus omit it).
Prudence is often cited as the first cardinal virtue, the 'charioteer of the virtues', and it seems strange that she is the only one of the four that seems to be mssing in traditional Tarot decks (except for the Minchiate). It is an idea that she is in fact hidden in the second trump - La Papessa or The High Priestess - as this seems rather more natural than a card dedicated to a female pope (Pope Joan). Her 'disguise' might be simply explained with the need to put a female counterpart to the Pope (Hierophant).