A "headphone plug" refers to the small connector or jack on the end of a headphone cable that you insert into a device, like a phone or computer, to listen to audio privately.
Sentences with «headphone plug»
As a result, I had to rely on the Note 5's bottom - firing speaker if I didn't have a pair of headphones plugged in. (tomsguide.com)
Note: They come with a typical 3.5 mm headphone plug, not a Lightning connector. (cultofmac.com)
Like most phones, things sound much better with headphones plugged in. (androidauthority.com)