A Docket number is your court-assigned case number for your citation.
It is always 10 characters long and begins with "T00".

If you have received your courtesy notice letter, your docket number is printed in the top-right:

Violations less than 14 days old may not be in our system - they do not have an assigned docket number and we cannot look them up.

A Docket number is different than your Citation #, which assigned by the agency that gave you the ticket (e.g. the CHP).