It refers to an important component within a keyword list.

Adding negative keywords to a particular campaign or ad groups can have negative impact on the ads that will be displayed online for a given term in the search queries. If you can filter out impressions that are otherwise not required, then negative keywords can help you in different ways like increasing ROIs, reducing CPC or cost-per-click, and can even help you in reaching out to excellent prospects. Adding negative keywords is similar to adding other forms of keywords. The major difference lies in the sign that is added in front of the keyword. The negative keywords can also be added while campaigning.