Translator's Note. Usually used in manga translations to convey additional information, though can be written elsewhere, such as fanmade anime subtitles.
See also: TLN or TL note.
"Bro, check out that gussy. Still pristine."
"Have you cleaned your gussy today, mom?"
Person A: Don't make me derp
Person B: What do you mean
Person A: Just start violently coughing at you. Derping