A stop word is filtered out because its meaning is insignificant in a search. These include: a, an, the, and, but, or, it, of, on, with, in, is, and are.
ONESearch ignores stop words in order to improve the search. However, in a phrase search, all stop words are included, except those at the end of a phrase. Ex.: the search man of the year has two English stop words of and the. If entered without double quotes (i.e., man of the year), results contain the words man and year. However, as a phrase search with double quotes ("man of the year"), results contain the exact phrase and don't exclude the stop words.