How does the entropy of a string of English text signify low quality?

Jeff Atwood recently tweeted a link to a CodeReview post where he wanted to know if the community could improve his "calculating entropy of a string" code snippet. He explained, "We're calculating entropy of a string a few places in Stack Overflow as a signifier of low quality."

The gist of his method seemed to be that if you count the number of unique characters in a string, that signifies entropy (code taken from PieterG's answer):

int uniqueCharacterCount = string.Distinct().Count();

I don't understand how the unique character count signifies entropy of a string, and how the entropy of a string signifies low quality. I was wondering if someone with more knowledge in this area could explain what Mr. Atwood is trying to accomplish.

Thanks!