AM (amplitude modulation) is a way to encode audio onto a radio carrier by changing the signal amplitude while the carrier frequency stays fixed.
It is older broadcast tech, but still useful for long-range radio because AM waves can travel far, especially at night.
Why night helps: during the day, the ionosphere’s D-layer absorbs a lot of medium-wave AM energy, so you mostly get local ground-wave coverage. After sunset, that D-layer weakens, so more signal reaches higher ionospheric layers and reflects back to Earth (“skywave”), which can extend reception to hundreds of miles.
Compared to FM, AM is usually more vulnerable to noise and static, but it takes less channel bandwidth.