It sounds like a subpoena to me. Those are the first 5 words of most subpoenas. Who sent you this document? If it is legitimate, (and it most likely is,) then you mist appear.
If you have doubts, bring it to an attorney for review.
Witness fees are $40 per day and 55.5 cents per mile.
You may want to sit down with an attorney to discuss.
Robert T. Kuehl
Kuehl Law, P.C.