Infrared is a means of using light to transmit a signal over distance.
The light frequency used in Infrared is above the range of visible light in the red end of the light spectrum.
IR is used for transmitting a signal representing sound in many Assistive Listening Devices (ALDs) or Assistive Listening Systems (ALSs).
There are many inexpensive IR systems designed for use with TVs, and many theaters use IR to broadcast to the IR equipped headsets that they will loan to patrons who want to hear the movie better.
IR has some advantages for use in assistive listening. It doesn't go trough walls and isn't affected by radio or electromagnetic interference.
Its disadvantages are that it's only suitable for indoor applications and it requires a clear line of sight between the transmitter and receiver.
Many people have strong opinions about whether FM or IR is better for transmitting assistive listening signals. The truth is, as long as the choice considers the advantages and disadvantages above, there is little difference in the quality of the signal.