The brief answer to this is that Leon Trotsky's main role in the October Revolution was that of military leader. Trotsky was the man who planned the overthrow of the provisional government that had been ruling the country since the February overthrow of the Tsar. It was Trotsky who devised the plan in which a group of soldiers would attack the Winter Palace in St. Petersburg as the government met there.
In addition to his role as military leader, Trotsky was also the second most important member of the Bolshevik Party. His place in the Party was second only to that of Lenin.
So Trotsky was a major player in the Bolshevik Party and was its military leader. He planned the October overthrow of the provisional government and led the Red Army through the ensuing civil war.