The koku was a unit of volume used to measure wealth in feudal Japan. Supposedly originally determined by the amount of rice needed to feed one man for one year, one koku was equal to 10 cubic shaku (the standard unit of length), in other words 0.28 cubic meters, or 278.3 liters. One koku of rice weighs about 150 kilograms.
During the Sengoku Period and the Edo Period, the wealth of feudal domains was measured by the amount of rice they produced in one year. Taxes were assessed in koku of rice, and the stipends of the samurai class were paid out in koku of rice as well.
The smallest domains in the Edo period were worth 10,000 koku per year, and the largest domain other than the shogun's own personal domain, Kaga, was worth 1,025,000 koku per year, earning it the nickname "Million Koku Domain."
Some domains, such as Matsumae in Hokkaido, did not produce rice, and other domains did not produce very much rice, but their wealth and there taxes were still measured in terms of koku of rice, so they might, for example, have to pay their taxes in an amount of timber equal to the value of 50 koku of rice.
The system of assessing wealth in terms of koku of rice during the Edo Period was called the "kokudaka" system. In the final years of the Edo Period, this system began to break down, and transactions increasingly came to be conducted in cash rather than in kind. This led to the increasing impoverishment of the lower ranking samurai, who found themselves having to buy things in cash, but still received their stipends in rice, which they thus had to sell, often into unfavorable market conditions, whenever they needed money. In practice this usually meant that the samurai would be forced to borrow money, usually at exorbitant interest rates, against the koku stipends they expected to receive at harvest time.
Further adding to the problem, the kokudaka system was built on extensive land surveys conducted by Toyotomi Hideyoshi in the 1580s, which had never been redone. The nominal wealth of domains, tax rates, and samurai stipends were fixed, but in actual practice the rice harvest dramatically increased over the course of the Edo Period, so a 50 koku annual stipend was worth a lot less in 1850 than it had been in 1650. This impoverishment of the lower rungs of the samurai class led many of them to support the overthrow of the Tokugawa shogunate in the 1860s.