Stalin's purges, carried out from 1936 to 1938, included the imprisonment and execution of Red Army officers, Communist Party leaders, peasant representatives, and dissidents deemed dangerous to Stalin's regime. There were few trials, and those that were often took the form of show trials, where the accused were falsely claimed to have plotted against Stalin's life or engaged in some act of sabotage. Hundreds of thousands of Soviet people died in the purges, which were part and parcel of Stalin's attempts to consolidate his power.
So are you just asking what happened? If so, the purges were about getting rid of people who might oppose Stalin's policies or who might become rivals to him. Stalin wanted to be the only real power in the USSR so he purged all of these people.