Negative keywords are specific words or phrases that help prevent your ad from showing to customers who are unlikely to click your ad.
You can use negative keywords to help prevent your ad from being displayed when a search query or other input contains your keywords but is irrelevant to your business. For example, if you sell tennis shoes, you don't want your ad showing to customers searching for "tennis courts." Add "courts" as a negative keyword to accomplish this.
You can add negative keywords at the ad group or campaign level, or even create a negative keyword list that can be shared across multiple campaigns.
When your ads are displayed to customers who are unlikely to be interested in your product or service, is not only a bad experience for the customer, but can have a detrimental impact on your campaigns as well. At the very least, those ads will artificially inflate your impression data and even worse, could cost you because of unnecessary clicks. If your ad is displayed and a user clicks that ad, even if in all likelihood they are not interested in your product, you still will be charged for that click.
By using negative keywords, you can reduce the possibility of showing your ads to searchers who are not looking for your particular service or product, saving them time and potentially saving you money.
If you sells shoes and you have specified "tennis" and "tennis shoes" as keywords, you can prevent your ad from being displayed in response to a search on "tennis courts" by simply adding "courts" as a negative keyword.
Similarly, it probably makes sense to include "shoes" as one of your keywords. But you might also want to add "brake" as a negative keyword, to avoid showing your ad to searchers who really need an auto shop.