Negative Keywords restrict your ads from appearing on queries that are not related to your business or service. Negative match is used with broad and “phrase match” keywords to prevent the AdWords system from adding words or phrases to your keywords that they deem relevant.
The below example demonstrates the need for -negative keywords. This advertiser does not sell automatic can crushers or instructions on how to build your own can crusher. Negatives should be added for -auto, -automatic, -build. Another option would be considering [exact match] for these keywords.
Search Query Performance Report
