A Yahtzee is earned when you have five ones twos threes fours fives or sixes at the same time. When you roll the dices the first time, hold the dices with the number that is rolled the most. (e.g. if you roll 2-1-5-3-2, hold the two twos) Roll the remaining dices again (1, 5 and 3) and hold the dices with the same number you chose earlier(2 in this case). After that, roll the dices the last time, and hope that you end up with 5 dices with the same number. (This is a Yahtzee) After 10 Yahtzees this achievement will unlock.