Plain English Meaning
The wing or tail surface is meeting the oncoming air from above instead of below, so instead of pushing up it pushes down.
Full Definition
An angle of attack in which the chord line of the airfoil meets the relative wind from above rather than below, producing a downward (negative) lift force. On a typical wing this occurs when the nose is pitched well below the relative wind, such as in a steep pushover or certain dive entries. On a horizontal stabilizer it is the normal operating condition in level flight, since the tail is designed to produce a downward force.