The Winning Phrase is displayed at the end of a match when you win against another player(s).
You can choose between several phrases. To change it, go to your Profile and tap on the green square on top of your Profile image.
You'll be able to see the phrases that can be used in this menu. The "free" pack is available for everyone, right from the start.
If you purchase the other two packs (Action or Premium), you'll unlock eight more phrases from which you can pick the one that fits you the best.
You can only use one Winning Phrase at the time. Choose wisely ;)
Whenever you win a match, the phrase will be displayed on the scoreboard for every player! Enjoy your moment of glory!