It's a card game. You had out all the cards in the deck to the players, and then you'll go around to each player, and they will put down a card in ascending order.
So it'd go like, Aces are 1s. So if you have two Aces you'd put them down (facing down), and say, "Two aces." Then the next person will say "One two." The idea of the game is to get rid of your cards, so you can put down as many as you want, but you have to be careful. Say it's your turn and you're number is 4. Say you don't have any 4's so you pick a card, lying it face down, and you lie. The trick is to catch someone in the lie by saying, "Bullshit!"
If they catch you in the lie, then you have to take all the cards in the pile. It's also fun to try and trick people, make them think you're lying when you're not, that why they have to take all the cards. The end of the game is whoever runs out of cards first wins, but it can take a while.
I hope that all makes sense! I'm not great at explaining things. It'll be easy to pick up when you see it in action!