World of Warcraft uses a system that can allow players to pay $20 in exchange for a token that has a gold value. Players can also use in-game gold to buy a token and use it to add game time to a player's WoW account. However, Blizzard recently added a new use for the token that can now be applied to its other games like Overwatch, Heroes of the Storm and Hearthstone.
Now, using current gold to token exchange rates, if you farm 62,393 gold in World of Warcraft, you can buy a token on the auction house and then exchange it for a $15 credit on your Battle.net account. That credit then can be used for Overwatch loot crates, heroes of the Storm character unlocks, or Hearthstone card packs.
The token system was originally instituted with a two-fold purpose: allow players to add subscription time using their gains from World of Warcraft, and to cut down on people using illegal gold sellers to pay cash to get gold.
If any of the explanation is confusing, Blizzard released a video explaining the process and the addition of the trade option for Battle.net credit.