A "command economy" operates a lot like the term sounds: it's an economy where someone's always telling it what to do. Specifically, the government.
This setup involves a central authority determining all aspects of economic decision-making. So bureaucracy within the government determine production and investment goals. They figure out prices and wages. All the functions that might otherwise get fulfilled by private companies and free markets become governmental responsibilities.
The Soviet Union and the Eastern Bloc countries of the post-WWII era represent the most famous command economies in history. However, these situations come up from time to time even in countries generally committed to free markets, most often during times of war.
In emergency situations, the government tends to take over more aspects of the economy, such as Great Britain or the U.S. during the World Wars (or even during other emergencies, like the Great Depression). These scenarios see governments implementing things like production quotas or rationing in an attempt to focus resources on a narrow goal...like winning the war. In most of these cases, the government relinquishes control after the emergency is over.