When you create an add for Google, you let Google know what type of product or customer you are looking for. Through Google Adwords, you pay Google to send people to you if they search for a particular word (known as a keyword).
Negative Keywords are words you let Google know that represent a type of customer that you DON’T want sent to you. Or, a type of keyword that prevents your ad from being triggered by a certain word or phrase. Your ads aren’t shown to anyone who is searching for that phrase. This is also known as a negative match.
For example, when you add “free” as a negative keyword to your campaign or ad group, you tell Google Ads not to show your ad for any search containing the term “free”. On the Display Network, your ad is less likely to appear on a site when your negative keywords match the site’s content.