CW stands for continuous wave, an electromagnetic wave of constant amplitude and frequency. In radio operations, it refers to the transmission method where this carrier wave is simply switched on and off to form Morse code characters. Because it concentrates all transmitter power into a single narrow frequency, it can cut through heavy interference and propagate globally on minimal wattage.
Heard on the job
The band is too noisy for voice, so switch to CW and see if you can raise the maritime net.