modulation
A radio wave at, say, 100 MHz is just a pure tone, a perfectly steady oscillation, and a steady tone carries no information, like an unchanging hum. Modulation is the act of writing your message onto that wave by deliberately varying one of its features in step with the message. The high-frequency wave is the carrier, chosen because it travels well through space, and the low-frequency message, your voice or data, rides on it. Demodulation at the receiver reads the message back off.
There are two classic ways to vary the carrier, and they name the two oldest broadcast bands. In amplitude modulation (AM), you vary the height of the carrier wave up and down to follow the message: loud moments make tall waves, quiet moments make short ones, and the receiver recovers the message by tracing the envelope of the wave. In frequency modulation (FM), the carrier's height stays fixed but its frequency wiggles slightly higher and lower in step with the message. The key practical difference: AM's information lives in amplitude, which is exactly what electrical noise and lightning corrupt, so AM is noisier; FM hides the information in frequency, which noise disturbs far less, so FM sounds cleaner, at the cost of using more bandwidth. Digital modulation extends the same idea, switching the carrier's amplitude, phase, or both among a set of defined states to send bits.
Modulation is what makes wireless communication possible at all: it lets a slow message travel on a fast carrier, lets many stations share the air by each using a different carrier frequency, and lets a small antenna radiate efficiently because the carrier's wavelength is short. Honest framing at orientation level: there is no free lunch. Squeezing more information onto a carrier always costs bandwidth, power, or robustness against noise, and every real scheme is a deliberate trade among those three. AM and FM are the simple analog ancestors; modern systems pile on sophisticated digital schemes, but the core idea, vary a carrier to carry a message, is unchanged.
An AM station at 1000 kHz carrying a 1 kHz musical note sends a 1000 kHz carrier whose height swells and shrinks 1000 times a second. An FM station instead keeps a constant height but nudges its frequency up and down 1000 times a second, which is why FM survives static far better.
AM varies the carrier's height; FM varies its frequency, which resists noise better.
There is no free lunch: sending more information always costs more bandwidth, more power, or less noise immunity. AM puts the message in amplitude (easily corrupted by noise); FM puts it in frequency (more robust but wider).