What is the Tsuka on a katana?

I'm curious, can you explain what the Tsuka on a katana is? I've heard it's an important part of the sword, but I'm not entirely sure what its purpose is or how it's used. Is it the handle that the swordsman grips? And if so, what materials are typically used to make it? I'm fascinated by the intricacies of traditional Japanese swordsmanship and would love to learn more about this fascinating aspect of the katana.