Stop Words
The stop word list defines words that the search engine should ignore, either completely or largely. The reason for using this is that search phrases may often contain words that are not relevant for the search function. For example, if a customer searches for Jacket made from leather, the key terms are jacket and leather; the words made from can be ignored.
The checkbox Relevant decides, whether the term is a function word (checked) or a stop word (not checked).
Function words (the, what, who, a, an, etc.) do not contain information that is relevant to the search process, and can be largely ignored.
Stop words, on the other hand, are words that you do not want to be found. These include expletives or terms that are subject to trademark laws.
Please be aware, that e.g. the terms “with” or “without” should not be regarded as irrelevant. The search for “milk without lactose” should not be reduced to the combination “milk lactose” because in the worst case this will produce all non-lactose free milk products. The lactose-free products might also be among the results, but as a result of luck, not clever use of stop words.