NOT
The NOT operator tells the database that any results that include a specific word should be excluded from your results. We don't use the NOT operator very often, but it's very helpful to have when your search results are littered with things that technically meet the terms of your search but aren't relevant to your research.
Think about this search: School NOT Fish
This search will return results that do include the term School but don't include the term fish. You might use this search if you are looking for articles on education but your results have a lot of sources about marine biology. The left circle below represents the results from the search for school. The right circle represents the results from the search for fish. The only the area of the left circle that doesn't overlap the right circle is shaded to show the results of the search School NOT Fish.
This is a smaller number of results, but if what we're looking for is information on education and not marine biology, these results are much more likely to be relevant. Removing terms from your results can have unintended consequences. Be prepared to iterate on your searches when using this operator.