This option allows you to set a value for which the game will ask you if you are sure that you want to make a Gold purchase. The value on the right represents the minimum value of Gold on a purchase for these confirmations to appear. Gold purchase values that are more than this number will offer a confirmation prompt before purchasing, whereas anything less will not.
Using a weapon valued at 800 Gold as an example, if you have the Gold Spend Limit on 100 and tap the purchase button, a confirmation prompt will appear asking if you are sure that you would like to make the purchase, which you will then need to tap the “OK” button to confirm the purchase. If you have the Gold Spend Limit on 1000 and tap the purchase button, the purchase will be made instantly.