When it is your turn, try to match the card on the discard pile, either by color, number, symbol. If you can't, you must draw a card from the pile. If it can be played, you can do so in the same round. Otherwise, it's the next player's turn. If you only have 1 card left, you must press the UNO button, otherwise you have to pick up 2 cards as penalty. There are also some special cards that let you make special actions. Full rules here: https://en.wikipedia.org/wiki/Uno_(card_game).