Stereo audio is where you receive a slightly different audio signal into each earpiece of your headphones. If you have full hearing and you listen to music or a film through headphones you may notice that some of the instruments or sounds only come through one side of the headphones. This will give the audio of your music or films a 3d quality as if the sounds are coming from all around you. The issue here is that some people have a hearing impairment which only affects one ear. If you are deaf in one ear and using headphones you are not going to be able to hear the full range of audio generated by a stereo signal and may miss out on some of the instruments in music or the sound effects in films.
A strategy used to combat this is to join the two separate audio signals together into one signal (a mono signal) which contains ALL the audio information and then send it to all speakers. This is called mono audio.
The Windows and Apple Mac operating systems and both Apple and Android phones allow stereo signals to be converted to mono through their accessibility settings.
Below we see the Windows accessibility settings with mono audio turned on.