tedster - 7:31 pm on Feb 19, 2013 (gmt 0)
It only means that your titles are probably not in the right way that people find interesting.
In particular, it means their title rewrite algorithm finds that the titles not optimal for a particular keyword. You often see rewriting on a query for site:example.com. What is a person making that search looking for? That's the theoretical question the algo is asking - and rewriting would not be for the site owner but for someone in the general public.
I think it does not affect your traffic in any significant way.
For the site:example.com query, I agree because it is not a common query for the general public. However, if the algorithm starts rewriting the title for a high volume query, it can sometimes have a big effect on click-through. When they get it right, it helps.