It means that the prophet s.a.w didn't say bismillah at the beginning of the surah, and that's the way it's remained. It's proof that the Sahaba didn't add anything when putting the Qur'an together into book form, even if they thought they saw a mistake, every surah has bismillah...you'd think they would think the surah was meant to have one aswell and put bismillah at the beginning, but they left it exactly as the prophet s.a.w recited it..not adding even a letter.