They were intended to transform the Soviet Union from a peasant society into an industrial superpower. His development plan was centered on government control of the economy and included the forced collectivization of Soviet agriculture, in which the government took control of farms.
Millions of farmers refused to cooperate with Stalin's orders and were shot or exiled as punishment. The forced collectivization of Soviet agriculture also led to widespread famine across the Soviet Union that killed millions.
Stalin killed 15,000 army officers as a purge of soldiers/people that he did not trust.
He thought he would win easily and it would be a short war, but because his army was in such disarray, it took years.
Because of Stalin's refusal to believe Germany had been preparing to invade the USSR, they were unprepared and quickly lost ground early on.
It stopped the German advance into the Soviet Union and marked the turning of the tide of war in favor of the Allies.