Modulation is change.

So an AM signal modulates the amplitude of the signal. So I would give off a constant 900Khz tone, and make it more or less powerful on a scale of 20Khz for a audible signal. This is the classic analog am radio.

An FM radio signal modulates the carrier frequency by small amounts. Here I would transmit a higher frequency, let's say 90MHz and slightly move this frequency back and forth while keeping the amplitude constant. (Its slightly more complex, but these days nearly all FM signals are digital data streams. )

An unmodulated carrier signal does neighter. It is a constant power, constant frequency signal used for triangulation and positioning. You would use multiple stations and multiple frequencies. Each station is at a known location. Given a directional antenna you can find the direction of each. If you have the angle to two stations, their location, and your altitude you can use this to find your location on earth.

Unmodulated signals are simpler, but do not provide you any data besides their angle, and relative signal strength.

Modulated signals can be more complex, but can include data such as the time, or allow for multiple signals in a near band.