In FM voice the transmitter's frequency is pushed up and down in step with your voice while its power stays constant. It is the everyday mode of VHF and UHF: handhelds, car radios, repeaters and local simplex chats. It sounds clean and quiet, and it is built for short, reliable, local contacts rather than long distances.
Most noise (static, ignition, lightning) changes a signal's height, not its frequency. An FM receiver clips off height variations and listens only to frequency, so a decent signal arrives almost noise-free. The price is bandwidth: roughly 10 to 16 kHz, several times an SSB signal.
How far the frequency swings is the deviation, and it follows how loud you speak. Too much and the signal spills into neighbouring channels and sounds distorted.
Louder or closer to the mic = wider frequency swing (deviation).
DeviationToo wide
Two behaviours surprise newcomers:
Capture. When two signals share a channel, the receiver locks onto the stronger and the weaker vanishes, rather than both being heard.
All or nothing. Above a threshold the audio is quiet and full. Below it, noise rushes in quickly. FM is poor at digging a weak signal out of the noise; SSB and CW do that far better.
Illustrative. In FM, a signal only a few dB stronger wins outright; the weaker one disappears rather than fading under it.
In practice
Speak normally, a few inches from the microphone. Shouting or pressing the mic to your lips over-deviates.
Match the repeater's setting. Many radios offer a narrow FM setting, roughly half the deviation. A mismatch sounds too quiet or distorted.
Set squelch just above the noise, so the speaker stays silent between transmissions. See Squelch, CTCSS and DCS.
Use the lowest power that works. Handhelds often have a high/low switch.
Listen first. Capture means a stronger station will wipe out a conversation you could not hear.
Hold the antenna upright, matching the vertical polarization of nearly everyone else.