Currently Rewarded Ads are a mobile only feature due to restrictions on other platforms which prevent us from showing rewarded ads there.
Mobile players have the option to watch an ad every 8 hours or so in order to earn a small reward. The Rewards are small enough that they won't have an impact on Leaderboards.
If you are a PC player and you would like to take advantage of the Rewarded Ads, we encourage you to link your game account to a mobile device such as a phone or tablet so you can see the Rewarded Ads.