Plain English Meaning
Ice building up on the spinning propeller. It changes the shape of the blades, so they push less air, the engine has to work harder, and the uneven ice can make the propeller shake.
Full Definition
The accumulation of ice on the blades and hub of an aircraft propeller when flown through visible moisture at temperatures at or below freezing. Ice forms first and most heavily near the propeller spinner, where blade speed is lowest, and reduces propeller efficiency by disrupting the airflow over the blades, decreasing thrust, and creating imbalance and vibration as ice sheds unevenly.