Originating from the Paradox Forum user Blue Emu, who was notorious for doing this, it means someone on the internet has answered a question while you were writing the answer yourself. Commonly occurs on forums.
(5 minutes ago) Person 1: How do you spell the fear of long words?
(3 minutes ago) Person 2: Hippopotomonstrosesquipedaliophobia
(3 minutes ago) Person 3: Hippopotomonstrosesquipedaliophobia