The StopWords.txt file contains a list of words to exclude from the index words. It is used to filter out common words such as "and", "the", etc.
To add a word to StopWords.txt file, first mark the word in the Index that you wish to add to the StopWords.txt file, and then click on this menu item. The marked word is added to StopWords.txt file and all words are then unmarked.
The results of adding new stop words appears immediately. From now on, all searches (including the current one) are processed according to the new stop words list.
In general, it is NOT a good idea to add many words to this file, since the effect of removing words from the Index may be puzzling in later searches.