Passing only happens on the first turn of a round. If you go first, you have the opportunity to take the upcard from the Discard Pile or to “Pass”, which provides your opponent with a chance to choose whether to take the upcard; if they decline, you automatically draw from the StockPile. If your opponent goes first, they have a chance to take the upcard first, and if they “Pass,” you are given the opportunity to take it.