The nail(s)--represent the scars that were in Jesus' hands when He showed them to the disciples to prove it was really Him.
25So the other disciples told him, "We have seen the Lord!"
But he said to them, "Unless I see the nail marks in his hands and put my finger where the nails were, and put my hand into his side, I will not believe it."
26A week later his disciples were in the house again, and Thomas was with them. Though the doors were locked, Jesus came and stood among them and said, "Peace be with you!" 27Then 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."