Yermak Timofeyevich was a prominent Cossack leader and explorer in the 16th century. He is best known for his role in the conquest of Siberia, where he led a group of Cossacks into the region, expanding the territory of the Tsardom of Russia. His expeditions began around 1581, and he played a crucial role in establishing Russian presence in Siberia. Yermak's campaigns were marked by battles against the Siberian Khanate, particularly against its leader, Kuchum Khan. His success in these conflicts helped pave the way for further Russian exploration and colonization of Siberia, significantly impacting the region's history and development.