This means that they have looked at the problem, but have attempted it 0 times. This is opposed to (. 0), where they have never even loaded the problem. Also, downloading a hard copy of the whole set counts as looking at the problem, so a student with an entire row of (0 0) has probably downloaded a hard copy.
I would guess that your student thinks (s)he has submitted answers. But all they have done is hit the enter key, which defaults to the Preview Answer button, not the Submit Answer button. So they think they submitted something, but they haven't.