"War communism" was the system that was implemented by the Soviets during the Russian Civil War that followed the Bolshevik Revolution. It was in place from 1918 to 1921.
There were a few important aspects to war communism. First, it allowed uncontrolled printing of money in an attempt to create enough money to buy things for the army. This led to hyperinflation. Second, it nationalized practically all manufacturing and even most retail selling. Third, it forcibly took agricultural products from farmers and imposed strict price controls on upon those products. Finally, it involved forced labor for civilians as well as for the military.
In this way, war communism was a very strict system of government control of the economy for the purpose of maintaining the Soviet war effort.