The signet ring was a symbol of kingship. It was a finger ring made of gold or silver with an engraved stone. People used it to seal or approve official documents.
This image shows the divine authority that God gave to Zerubbabel. It assured God’s people that the Lord was still involved in political life, even after the failures of the kings of Judah.
Calling Zerubbabel the Lord’s signet likely renewed hope for the Messiah (God's chosen leader), since Zerubbabel was a descendant of King David. However, this promise ultimately points beyond Zerubbabel (see study note on Haggai 2:21). It points to one of his descendants, the Messiah, who would come later (see Matthew 1:12; Luke 1:32–33; 3:27).