https://www.youtube.com/watch?v=08mwXICY4JM
Large grids keep alternating current near a setpoint — 60 Hz in North America. When generation cannot meet demand, frequency falls; fall far enough and cascading trips can black out the system.
During the February 2021 freeze, ERCOT frequency plunged as gas, coal, nuclear, and wind units failed in the cold. Forced load shedding stopped a deeper collapse within minutes of under-frequency thresholds.