Plain English Meaning
Pilots working toward becoming flight instructors. They are held to a higher demonstration standard than other pilots because their job will be to teach.
Full Definition
Pilots who are training for, or being evaluated on, the requirements to earn a Flight Instructor Certificate (CFI). They are typically commercial pilots learning to teach flight maneuvers from the right seat and to demonstrate maneuvers — including certain advanced stall recoveries — to a higher standard than required of a private or commercial pilot.
Context Anchor
In the Airplane Flying Handbook, this phrase usually marks material aimed at pilots learning how to teach a maneuver, not just perform it themselves.