The Supply Chain is a daily event in Battle Islands where each new day upon returning to the game you are offered a free spin of the Supply Chain. Starting the Supply Chain will start the movement of the chain and after a few moments will again come to a halt. Wherever the marker points at along the Supply Chain, the resource (Either gold or supplies) will be awarded. The Supply Chains prizes of either gold or supplies come in different quantities and when awarded will be transferred directly to your total of that resource.
After the free daily spin, there is for the remainder of the day the choice of playing the event again over and over at a cost of 5 gold for each new Supply Chain spin. This can be beneficial and a cheaper option to gain supplies or gold than to purchase them directly from the treasury. However it is never a guarantee you will get what you want.