This question is to test if you are capable of identifying the gray area between two very similar situations. He wants to check if you can differentiate between the ‘good’ and the ‘bad’.
For making an impressive  answer, try to explain the following:
  1. There is a thin line  between the two similar but contradictory words – confidence and  over-confidence; where one can make your career whereas the other can ruin it. 
  2. Having confidence is “to believe  in one’s own self and to know one’s own skill, potential and value”; whereas having  over-confidence is “to have unrealistic faith and belief on one’s  capabilities”.
  3. You prepare for coming to  the interview – this indicates that you built confidence in yourself to face  the real situation. On the other hand, if you are over-confident, you will go  without any preparation. That’s called ‘being unrealistic’.
4. “Confidence AWAKENS one’s  soul; whereas over-confidence makes a person BLIND to the facts and reality of  the situation.”

Now, it’s time for 'participation':
*Assume that you are sitting in the interview room now.
*You have been asked to ‘tell something about yourself’. Go ahead; give it a try on how to answer the same. Post your own answers and get them reviewed .

'Knowing oneself' is a pre-requisite for building confidence. But over-confidence leads a person to have false belief that he knows everything including himself.