Okay kiddo, listen up. Thomas the Apostle was a friend of Jesus. He was one of the guys who followed Jesus around and learned from him. But one time, Jesus died and Thomas didn't believe that Jesus came back to life. He thought it was too crazy to be true.
Later, when Thomas saw Jesus himself and felt his wounds, he knew for sure that it was really him. He said “My Lord and my God!” because he realized that Jesus really was the son of God.
So Thomas is often called “Doubting Thomas” because he didn't believe at first, but he is also an important figure because he helped spread the word about Jesus and became a believer himself.