Closed-class words, such as articles, pronouns, and conjunctions. In English, words such as a, is, she, the, that, and will are filtered out of a search query. For example, in the query the red house, the search discards the and searches for the terms red,house.
But this feature is disabled if stop words are part of a phrase surrounded by quotes, then stop words are not discounted “the red house” searches for all 3 words in that exact order.