The presence of such a keyword prevents the display of an ad. For example, if your keyword is "red widgets" and the word "free" is specified as a negative keyword, then your ad will not be shown to a user who is looking for free red widgets.

A keyword matching option referring to keyword term or terms that are used to NOT trigger ads, i.e. sort out irrelevant search queries

Words that when assigned to a keyword, prevent the ad impression. They are critical in order to direct a campaign to a specific target.