tickets.is_hold records whether a ticket has been placed on hold by an agent. The on hold state has been superceded with the addition of Pending Status. This article is only applicable if on Deskpro version 2019.2 or older, any newer version of Deskpro you can report on Pending status with this guide.
The value of is_hold is 1 when the ticket is on hold, and 0 otherwise.
If you wanted your report to return only tickets that are on hold, you'd include this in your DPQL:
WHERE tickets.is_hold = 1
Note that is_hold is separate from tickets.status - a ticket that's on hold will always have a status of awaiting_agent.