Thomas was one of the Twelve. He was called Didymus. He was not with the other disciples when Jesus came.
References for John 20:24
So they told him, "We have seen the Lord!" But he said to them, "First I must see the nail marks in his hands. I must put my finger where the nails were. I must put my hand into his side. Only then will I believe what you say."
A week later, Jesus' disciples were in the house again. Thomas was with them. Even though the doors were locked, Jesus came in and stood among them. He said, "May peace be with you!"
Then he said to Thomas, "Put your finger here. See my hands. Reach out your hand and put it into my side. Stop doubting and believe."
Thomas said to him, "My Lord and my God!"
Then Jesus told him, "Because you have seen me, you have believed. Blessed are those who have not seen me but still have believed."
Jesus did many other miraculous signs in front of his disciples. They are not written down in this book.
But these are written down so that you may believe that Jesus is the Christ, the Son of God. If you believe this, you will have life because you belong to him.