Agentonly Fields are custom fields which can be used by agents for a ticket. It is not available in ticket form nor visible by user who raised this ticket. It has all the same field types as custom fields.
It is visible once an agent open a ticket. If agent has access to change agentonly fields, he can edit and change it.
It can be used for storing some information like phone number, important note, etc. which often needed for quick overview of ticket.