Plain English Meaning
These are the updates pilots must give to controllers during an instrument flight. Some you always have to make; others you only make when ATC is not watching you on radar.
Full Definition
Position, status, or condition reports that pilots are required to make to Air Traffic Control under specified circumstances during IFR flight. Some reports are mandatory at all times (such as leaving an assigned altitude, unforecast weather, or equipment malfunctions), while others are required only when not in radar contact (such as reaching a reporting fix or estimating arrival times).
Context Anchor
You encounter ATC reports during IFR flying, , altitude changes, , and any time ATC needs -provided information to a safe picture.