Stockpiling, or stocking, is the act of changing pure currency to liquid assets which can be used to hold value for a later date. This is normally done by farmers acquiring bushels, or oilers producing oil (although to a lesser extent), or other countries using the funds produced by other strategies to acquire either bushels or barrels, putting these items on the public market at prices not intended to sell, and repeating this process--while buying more and more items, and placing those on the market as well, and amassing a very large stockpile--until the time the items (known as 'stock') are sold (called destocking).
Stocking was made necessary by the presence of a hard limit of the amount of cash a country could have on hand, in the old Earth:2025 game. This hard limit (called the $2 billion bug) was inadvertently imposed by the game programmer's use of a signed INT, rather than a BIGINT, to control how many measurements of the game were calculated. This meant that every country could only have a maximum of $2,147,483,648 on hand (2^31 plus the signed bit) at any time. Any additional amount of cash acquired would either cause the cash on hand to reduce to exactly $2,000,000,000 when a public market transaction was involved, or reduce to $0 when a private market transaction was involved.
Players found that they could get around this limit by buying up cheap items on the public market, and then selling them back at a later date (while keeping more and more of these items in store, as their cash reserves allowed), in batches so as not to hit the cash limit. This led to the public market becoming more of importance than what the game's admin intended, but this 'feature' was left in place to garner another method of interaction among the players. Since bushels and oil were normally the cheapest to buy, and they didn't affect expenses, they would become the main items to acquire and store value. Due to the normal volatility of the oil market, however, bushels would become the main item of stock among the players. Since very few people had been able to acquire over 2 billion of either of these items, this method of gathering more value than the game originally allowed became the main method of obtaining higher and higher networth, and many strategies were created to increase the amount of bushels either produced or stored for the hopes of having more and more bushels on hand when it came time to sell them off.
The public market also became the place to store this stock, rather than leaving it all in the country. This was achieved by simply putting the stock on the public market at a high price so it--hopefully--wouldn't get sold. This meant that countries who were attacked while their stock was on the market didn't lose their stock. However, due to the time limits of how long a batch of goods could remain on the market before being returned to the country, there was always a possibility that a country's stock could return home before the owner of the country would be able to log in and put it back on the market. This eventually resulted in the rise of suiciding, where countries would be attacked solely for the purpose of either stealing their stock or destroying it. As long as a country's stock remained on the market, it would be safe from all forms of military attack. Also, putting a batch of bushels on the public market reduced food decay.
In Earth: Empires, the $2 billion bug was intentionally coded in to keep the public market viable. However, this 'bug' only affects the amount of cash a country can have on hand; unlike in Earth:2025, where pretty much everything was affected. Therefore, the method of stocking and destocking to either gain a higher networth or help in times of war remains the main method of acquiring value.