Zeniba's seal is a precious object owned by Zeniba. The seal is protected by a protective spell. The seal is not seen until later in the film. It seems that the spell was also broken when Chihiro held the seal in her hands. The protective spell causes the thief to die and can only be broken by love.
The seal was stolen by Haku. He acted on behalf of the Witch Yubaba. Because of this order, Haku (as Dragon Ryuu) was almost killed. The dragon spits out the seal with the black worm-like creature after Chihiro has fed him with the herb dumpling by the river spirit.
Then Chihiro brings the seal back to Zeniba and asks for forgiveness in place of Haku. As she takes the seal in her hand, she notes that the wards of the seal have been broken. Chihiro apologizes to her for crushing the black worm. Then Zeniba laughs brightly and explains that the worm was a parasite implanted into Haku by her sister Yubaba in order to control him.