‘On that day,’ declares the LORD Almighty, ‘I will take you, my servant Zerubbabel son of Shealtiel,’ declares the LORD, ‘and I will make you like my signet ring, for I have chosen you,’ declares the LORD Almighty.
A singet ring was a way to sign documents – a king or governor would have a uniquly designed symbol the would be on a ring, and that would be applied to a wax seal to make an impression. Hence a document or decree was “signed and sealed.”
It was valuable, and could be entrusted to another to use “in the name of” the actual owner.
Background to this text is important. In Jeremiah 22:24-30 the next to last King of Judea, Jehoiachin is said to be rejected, as if the Lord removed him like a signet ring and tossed him away. The end of the passage says that no descendant of his will sit on the throne.
So when his grandson Zerubbabel is given this message, it is a reversal of the judgment previously given.
Christians would read that as the nature of the Gospel – the curse is reversed. There was nothing in particular about Zerubbabel to merit the restoration – it would seem to be grace not merit.