Plain English Meaning
A way of flying on instruments where, for each thing you're trying to hold steady (like altitude, heading, or airspeed), one gauge is your main reference and the others are there to back it up.
Full Definition
An attitude instrument flying method in which the pilot identifies, for each maneuver, one instrument that gives the most direct indication of a desired performance value (the primary instrument) and other instruments that back up and refine that indication (the supporting instruments). The primary instrument for a given parameter is the one that shows the value being held constant most directly; supporting instruments confirm the and help the pilot the value precisely.