The Mystery Box can be purchased in the Market under the "New" tab. It costs 5 Wine Cash and contains an assortment of items that can only be obtained by purchasing the box. They are not available otherwise in the Marketplace. When you purchase the Mystery Box, you place it in your vineyard. Click on it to open it and see what you've gotten. A message will tell you which item you've received and that it has been placed in your gift box. To use the item, open your gift box; you will be given the choice to use it or sell it from the gift box.
If you want to be surprised by purchasing the Mystery Box, you will be to some degree. Items are provided randomly. But there is a limited number of items available and if you want to know the possibilities, click on the next tab to see what you MIGHT get. If you want to be surprised completely, then do NOT click on the next tab as the items you might get are revealed there.