The phrase "headphone output" refers to a connection or port on a device (such as a phone, computer, or music player) that allows you to connect your headphones and listen to the audio or sound from that device privately. Full definition
At the front is an input for the setup microphone and a virtual surround sound headphone output. (digitaltrends.com)
Its speakers are also only capable of mono sound, though headphones output in stereo. (dealspwn.com)
There's also headphone output on the unit itself for playback without any latency, a must - have for those who rely on the best in sound recording quality and need to have everything set up perfectly. (windowscentral.com)