When Stalin took over the Bolsheviks he wanted to make sure that Russia was rapidly industrialised. This didn't just mean changes in the towns and cities but also major changes in the countryside. Stalin wanted to increase food production to avoid any more of the bread queues that formed during World War One and to ensure that in the event of another war that the soldiers would not starve.
As part of his Five Year Plan, he started the policy of collectivisation. He did not believe that land in the countryside should be privately own and so started to take ownership of all the land to create collective farms. This meant that peasants had to give up any land they owned, or at the land they worked and join large collective farms with other farming families. They had to pool together their resources, particularly if they had mechanical equipment such as harvesters and tractors which were scarce at the time. By 1939 90% of all the land in Russia had been collectivised. Stalin's plan was that the large farms would grow much more and be much more productive than smaller farms. The government would technically own anything grown on the land and could take what they wanted when they wanted.
How did this work?
Initially, Stalin asked the peasants to join together voluntarily. The peasants ignored this and refused to collectivise, this was in large part because Stalin did not give any information about how much would be taken from the farms, if the farmers would receive any pay for this and if they would have any rights over their own property. There were food shortages in 1928 and the police seized food from the peasants. In 1929, Stalin made this compulsory and in reaction a lot of the peasants burned their crops and killed their animals so that the government could not seize them. The result of this was intense famine, particularly in Ukraine. Peasants began to starve, and the government continued to seize everything they had. About 2-3 million peasants died between 1932 and 1933 and in extreme cases starving peasants turned to cannibalism.
As well as peasants dying from starvation, other peasants were killed and sent to Siberia. The government started a campaign against a group of people called the "kulaks" (rich peasants). Any peasants who were seen as having more than the average person could be arrested and they wealth they owned taken. Many of the people arrested weren't rich, they simply had better land than others and sometimes they were reported by their jealous neighbours who wanted some of their stuff!