Plain English Meaning
It is the FAA's official rulebook that lists exactly what a pilot has to know and be able to do to earn (or keep current) an instrument rating. Examiners and instructors use it as the checklist for testing.
Full Definition
The FAA publication that sets the official knowledge, risk management, and flight proficiency standards a pilot must meet to be issued an instrument rating, and that examiners use as the basis for the instrument rating practical test (checkride). It also serves as the reference document for instrument proficiency checks (IPCs), specifying the tasks and performance standards an instrument-rated pilot must demonstrate to remain current and proficient.