The Five Year Plan (I assume that you are asking about the first of these plans) was Stalin’s plan for the Soviet economy for the years 1928 to 1932. The Soviet Union was a centrally planned economy. The government decided how the economy would be run rather than allowing market forces to influence the economy.
In 1927, Stalin felt that the Soviet Union was not growing fast enough. He felt that it was far behind the nations of the West and that, therefore, it needed to grow very rapidly in order to avoid being destroyed by them. This rapid growth was the goal of his Five Year Plan.
There were two main ways that this was to be achieved. First, agriculture was to be collectivized. This meant that land would be taken away from those who owned it and made into huge collective farms owned by the state and run by the people. Second, heavy industry was to be promoted and indeed mandated. If there were not enough volunteers to work in factories, people were to be forced to do so. Industrialization was so important that any measures were to be taken to allow it to happen.
Thus, the Five Year Plan was the first plan for how to improve the Soviet economy.