Plain English Meaning
The propeller is spinning faster than it is allowed to spin. Normally the propeller's control system keeps RPM at the value the pilot has set, but something has stopped it from doing that, and the engine and propeller are now turning too fast.
Full Definition
A condition in which a constant-speed propeller's rotational speed (RPM) exceeds the maximum value allowed by the engine or propeller manufacturer, typically because the governor or pitch-change mechanism has failed to increase blade pitch enough to hold the selected RPM.
Context Anchor
Seen in constant-speed propeller operation, especially when monitoring the tachometer during takeoff, climb, , or a problem.