Frequency Modulation (FM) means varying a radio signal's frequency (instead of amplitude) to transmit useful information.
Some assistive listening devices, ALDs, and some assistive listening systems, ALSs, use FM to transmit the signal representing sound from a transmitter to a receiver.
Many movie theaters now transmit the soundtrack of the movie in a low powered FM signal throughout the theater. The theater will loan hard of hearing people a special receiver they can use to receive that FM signal and therefore to hear better. You can buy personal FM systems that you can use at home, in restuarants or in the car to hear someone you are with better ... or even to hear the television at home.
Now, even some hearing aids can receive FM with an integrated (built-in) or boot receiver.
There is even a special frequency ranges that are assigned for use by assistive listening systems. FM has advantages and disadvantages. It can be used outdoors, and it can transmit through walls -- there is an alternative transmission method, IR, that uses infrared light to transmit the signal that has different advantages and disadvantages.
Whether the signal is transmitted via FM or IR, the advantage of using an ALD to transmit the signal is that it overcomes distance and can eliminate background noise.