Flags are default labels in Freshdesk that appear in the ticket list view and inside ticket details page when any of the activities below occur:
New: This flag appears on the ticket when a response from the agent is still not sent to the customer. The 'New' flag lasts until the first response/resolution isn't overdue
Customer Responded: This flag appears when a customer responds to a ticket.
Overdue: This happens when the ticket has gone past its resolution time and yet not resolved.
Response due: This appears when the First Response by the agent is not sent in that ticket within a specific time according to the SLA.
These are default and hardcoded flags and cannot be removed or edited. Also, there is no option to filter tickets using these flags.