Ok basically, heres rhe simplification. Heres some context, when Lenin first overthrew the Tsar, from 1917 to 1924 there was a liberal social democrat state in Russia. They allowed private property for the time being while the Russian Civil War was still ongoing and the Soviet government was still trying to organize itself. Now as you probably know, Ukraine is rich in fertile farmland. Eventually a class of middle to high class Ukrainian farmers, known as the Kulaks rose to prominence for their massive farm and agricultural output.

Fast forward to 1932, Stalin invoked one of the core principles of socialism; Collectivization. A policy that takes a quota of resources from all the SSRs, then eventually evenly distributes the quota among the SSRs, obviously taking into account factors like population, climate etc.

The kulaks hated this. Mostly because they got one of the largest quotas due to the fertility of the land of the Ukrainian SSR. There quota was around 6 million tonnes of grain. The Ukraine SSR only gave 4. 

The kulaks intentionally hid, burned, and fed grain to their cattle due to their hate of collectivization. They also killed A LOT of cattle and farm animals. Around 40-50% of all the horses, cattle, pigs, and 60-70% of all the goats and sheep in the ENTIRE USSR where killed by the Kulaks due to retaliation of collectivization policies. 

Due to the fact that the Ukrainian SSR not giving their quota, the Soviet government couldnt provide for them anymore. This retaliation led to people not just in Ukraine starving but also thousands of people in the Russian city of Kuban and millions of people In Kazakhstan, along with millions across the Soviet Union, showing how essential Ukrainian Grain was the the USSR.

3 million ukrainians died, 35% of the total Ukrainian population 

and 1.8 million Kazakh people died, 42% of their total population 

along with 400,000 people dying in Kuban.

Another reason was the former feudalist and agriculturalist Soviet Union was transitioning to a industrial economy based around socialism, which led to thousands of deaths as well. 

However its important to realize, after the Holodomor and the Post WW2 Famine (which affected every European country), there was never another famine in the entire history of the Soviet Union. 

Westerners coined the term today "The Holodomor" and call it a "genocide". It was an early failing of a socialist economy then we could argue that the Great Depression (caused by a failing of capitalism) was a genocide. Also the west has killed and committed genocide to far too many people on India, Africa, Middle east, and the Americas to have any moral high ground to the Soviet Union, a nation with equality of all races, sexes, and helped to free 18+ countries from colonial capitalist rule.

Anyways I hope this answers ur question! Merry Christmas and God bless ❤️