There’s no limit to the amount of resources you can collect in the game. The Warehouse provides storage space and concealment for your resources, which are deposited in the Warehouse by default.
Concealed Resource Capacity refers to the amount of resources that your Warehouse can conceal. When your Town is under attack, those Concealed Resources will not be plundered. You can upgrade the Warehouse to increase the Concealed Resource Capacity.
For example, when you have 40,000 Food in storage and the Concealed Resource Capacity of your Warehouse is 30,000, that 30,000 Food will be concealed during the attack, and your enemy can only plunder up to 10,000 Food.