In radio systems the carrier is the transmitted electromagnetic wave that is modulated by some means to carry information. In AM radio the carrier wave is at a fixed frequency, but its amplitude changes according to the signal contained within. In FM the center frequency is known, but the signal’s frequency changes around this center point according to the information carried. More amplitude creates a wider deviation from the center frequency. There are other types of modulation that may change phase or other attributes. When a carrier wave is “demodulated” by a radio receiver the information contained within is pulled out (by removal of the carrier wave) and subsequently amplified. The frequency you tune your radio or television to in order to pick up a station corresponds to the carrier frequency.
FM synthesizers also use this paradigm to generate sound. In that case the carrier is usually the sound you hear, but it is modulated by other operators to change its timbre and other characteristics to achieve the desired sound.