The New Economic Policy, or NEP was a system of economic reforms, partly capitalist in form, that Vladimir Lenin instituted in the Soviet Union in 1921. The policy of War communism the direct conversion to a communist system had failed, and the NEP was introduced in 1921 as an emergency measure.
The NEP restored some private ownership to essential parts of the economy, especially farming. And replaced central planning whereby the government tried to plan the production and allocation of goods, by means of production quotas and instructions. With a kind of "state capitalism" whereby nationalised (government owned) industries were allowed to operate autonomously and for profit, in much the same way as private businesses.
The NEP succeeded in creating an economic recovery after the devastating effects on the Soviet economy of the First World War the Russian revolution and the Russian civil war. It was abandoned after Lenin's death in 1924. The NEP was only ever intended as an interim measure, and proved highly unpopular with Marxist purists in the Bolshevik party because of its strongly capitalistic elements, they saw the NEP as a betrayal of communist principles, and wanted a planned economy instead.
Lenin's successor Stalin re-introduced central planning and re-nationalised all industries and from the late 1920s onwards introduced a policy of crash industrialisation.