Ticket Sub-Statuses can be added to the Core Statuses (Awaiting Agent/Awaiting User/Pending/Resolved) that are available to tickets in order to better define what state a ticket is currently in. Any Sub-Statuses that have been created by your Admin will be available for selection in the Status drop down field on the ticket:

In order to select a Sub-Status, you must first choose the correct Core Status, and the appropriate Sub-Statuses will appear, with a tick showing which have been selected.

The appropriate Core Status and Sub-Status will show when viewing the ticket, and Sub-Status will show in the list of tickets.

Sub Statuses can also be used in Follow Ups or other features that allow you to automate your helpdesk.