At the start of every month, the explorers guild will send a special "treasure chest" to everyone who helped exploring Adventuria.
Inside this chest, you'll find some Fairy Dust. In the future, it might even contain gemstones and copper coins. The amount of loot is directly tied to your collected miles. Every explored mile should yield about 1g of Fairy Dust, but there's also a bit of luck involved.
Your mile counter acts as the base value, when the amount of loot is calculated. Let's say you have explored 100 miles. This would mean that your monthly chest will contain about 100g-120g Fairy Dust, depending on luck.
Additionally to these normal luck-based fluctuations, opening a chest can trigger a "quiet month", which will cut your profits in half, or you could trigger a "lucky hit", which will double your profits.
Your discovery chest works in a similar way as your daily chests, by the way: when you don't open it for a whole month, it will expire as soon as you get another one. So you should check your dashboard at least once a month to claim your well-deserved reward.