Plain English Meaning
The three core skills you need to fly an airplane using only the instruments: looking at the instruments in the right pattern, understanding what they mean, and then flying the airplane based on what you see.
Full Definition
In airplane attitude instrument flying, the three basic skills a pilot must develop and combine to control the aircraft solely by reference to instruments: instrument cross-check (systematically scanning the instruments), instrument interpretation (understanding what each instrument is telling you about the aircraft's attitude and performance), and aircraft control (using that information to make smooth, precise to or change attitude, , , and ).