Back to year 2006 when I was working for my first sphinxsearch project I was playing with stopwords files. Stopwords is basically a small set of highly frequent words you often don’t want to search for (like “I”, “Am”, “The”, etc). For most sphinx instances they only wasting index space and slower your search queries by finding all occurrences of these non-important words.
Say if you are searching for “when is jane’s birthday” you are actually looking to find documents with “jane’s birthday”, and you don’t really care about lot’s of documents (blog posts, news articles, etc) with only “when” and “is” inside.
Remove those high frequency words from search index is usually smart move and ages ago I’ve created two stopword file samples which I’m using by now.