You'll never have to use the Joker yourself, so don't worry too much about understanding what it does. That being said, Jokers have two effects in Caravan in Fallout New Vegas.  First, if you place a Joker on a number card, it'll remove all instances of that number from the field, excluding the card the Joker was placed on. So, if you put a Joker down on a four, all other fours will leave the table.  If you place a Joker on an Ace, though, all other cards of that Ace's suit leave the table, instead. Neither of these uses is needed in-game - New Vegas' AI isn't strong enough to warrant using them.