Plain English Meaning
It is the controlled descent from cruising altitude down to the height where the pilot can begin lining up for landing, flown by instruments rather than by looking outside.
Full Definition
An instrument letdown is the procedure by which an aircraft descends from the en route altitude to an altitude from which a landing approach can be made, conducted solely by reference to instruments and following a published instrument approach procedure.
Context Anchor
Seen in instrument approach discussions, especially when describing how an IFR pilot descends toward a civil airport before landing.