Power-up is a very useful tool that will help you a lot during the game. In order to activate the Power-up while you play, click on the "Golden Dice" located on the bottom left part of your screen.
This powerup rolls to bring one dice with the highest payout, no matter how many dice you may have held. For example, if you have held no dice, Golden Dice will give you a 6. If you have held 2 "threes", then the Gold Dice will give you one more "three" like displayed below.
Note: If you have held 4 of a kind dices and roll a Gold Dice, then it's certain that you'll score a Yatzy, which is the highest combination.