Please give the complete sentence. I think this is referring to how often words are used. "Fraction" means ratio.

Used by itself, 'fraction of times' generally means 'hardly ever' or very few times at all. For example, 'He only went to the doctor a fraction of times' means he only went to the doctor a few times or hardly at all. The same with your example. A word appears 'a fraction of times' means it hardly appears at all, or not that many times. But you have to be careful that there aren't more words that qualify or alter this basic meaning. For instance, if I say a particular word appears in a text 'a fraction of times compared to another word', then the meaning shifts from 'just a few times, or hardly at all' to 'some number a lot less than the other word, but it may still be quite a few.'