Plain English Meaning
Going lower than the lowest altitude the approach allows, before you have the runway in sight and the conditions you need to legally and safely land.
Full Definition
On an instrument approach, continuing the aircraft below the published Minimum Descent Altitude (MDA) or Decision Height (DH) without meeting the regulatory conditions required to do so. Under 14 CFR 91.175, descent below MDA or DH is only permitted when the aircraft is in a position to land using normal maneuvers and a normal rate of descent, the flight visibility meets or exceeds the value published for that approach, and the pilot has at least one of the specified for the distinctly visible and identifiable.