CW (continuous wave) is Morse code sent by switching a steady radio carrier on and off. Key down, the carrier is on; key up, it is off. Nothing else is varied, which is why it is the simplest mode to generate and still a strong performer when signals are weak.
CW: Morse by switching the carrier. PSK: data by flipping the wave's phase.
Why narrow means strong
Noise is spread evenly across the spectrum, so the more bandwidth your receiver lets through, the more noise reaches your ear. A voice signal needs about 3 kHz. A CW signal is a single tone being switched, so it fits in a filter a few hundred hertz wide, or less.
Noise is spread evenly across frequency, so noise power grows with filter width. A CW tone is one frequency: a narrow filter keeps all of it and throws noise away. Schematic.
Signal-to-noise gain vs 2.4 kHz0.0dB
Drag the slider: shrinking the filter from 2.4 kHz to 150 Hz keeps the whole signal but removes about 12 dB of noise, a gain you would otherwise buy with a much bigger transmitter. See Noise and signal-to-noise ratio for why signal-to-noise ratio, not raw strength, decides what you can copy.
Longer bar = wider signal. Log scale: every tick is ten times the last, so TV is far wider than it looks.
The rest of the picture
Efficient transmitting. All the power goes into the one signal that carries the message, and the transmitter needs no linear amplifier. Small, cheap, battery-friendly rigs work well; see QRP CW.
Crowded bands. Stations can sit a few hundred hertz apart, so many more fit than on voice, and a narrow filter lets you pick one out. Your ear and brain are excellent at hearing a steady tone through noise.
Speed costs bandwidth. Faster keying makes sharper changes, so a fast signal is a little wider. Keyed too abruptly, a transmitter splatters clicks across the band, so rigs soften the on and off edges.