1/ A user wants to get a virtual item or in-game currency (coins, in this game);
2/ The user goes to the store and watches a video ad to earn currency for free;
3/ When the video is over, the user gets X coins for free, while you get paid by the advertiser.
Monetise your app today: http://unityads.unity3d.com
Clients include Rovio, Hipster Whale (Crossy Road), Zeptolab, SEGA, LOTUM, Game Insight and many other successful developers.