The Warehouse is a building that determines the total amount of gold you can accumulate. Your maximum gold capacity is 150,000 (after getting the Warehouse) plus 30,000 for every level of the Warehouse. Bonuses (e.g. event rewards) can put you over this maximum, but hourly gold production stops once your Warehouse's limit is reached.
You also may not harvest gold from your farm if you have met or exceeded this limit. Once you reach or exceed the Warehouse's maximum gold storage, you must spend enough gold to be below the maximum amount before hourly gold production begins again.
(See the Building page for upgrade costs and cooldowns)