Mono audio is an Accessibility feature that makes sure, even if you're hard of hearing or deaf in one ear, you never miss a word, note, or sound when listening to your iPhone or iPad with a headset on. Typical stereo audio includes distinct left and right channels, so different ears get different sounds. Mono audio makes sure both ears get all the sound. You can also adjust the volume independently for either ear, so you everything from audio books to podcasts to songs to videos come in loud and clear.
Note: You can also use mono audio if a podcast or other audio file you're listening to has mistakenly put one panelist in the wrong channel, all panelists in only one channel, or not properly balanced the volume between channels.