Plain English Meaning
It is a grey line drawn on an approach chart between the visual descent point (or missed approach point) and the runway. It is a warning that obstacles stick up into the visual path to the runway, so once you leave the published minimums you have to fly the rest visually and watch out for those obstacles.
Full Definition
On an instrument approach chart, a grey shaded line is a charting symbol used to depict the visual segment of an approach procedure — the portion between the published missed approach point or visual descent point and the runway threshold, which must be flown visually. It indicates a known obstacle penetration of the visual segment surface, alerting that obstacles exist along the visual path to the runway and that below the published minimums must be made with continuous to the .