What does "herein fail not, and you have then and there this writ" mean?

Question

I recieved a summon in the mail to appear before the Grand Jury out of my state, on the same day I recieved the letter. I need help understanding what my question is about.

Answer 1

It means don't fail to appear, and bring that writ with you, so as to have it with you then and there, when you show up.