Plain English Meaning
Learning to fly by trusting and reading the instruments instead of looking outside, so you can keep flying safely when you can't see the ground or horizon.
Full Definition
The phase of pilot training in which a pilot learns to control an aircraft, navigate, and conduct flight operations solely by reference to cockpit instruments, without relying on outside visual cues. It develops the skills required to fly safely in clouds, low visibility, or other conditions where the natural horizon is not usable.