Negative keywords can have a positive impact on your campaigns because they help you filter out irrelevant searches. When you include negative keywords, you’re essentially requesting that the system does not show your ads against search queries containing those words. The Keyword Tool automatically gives you suggestions for negative terms, and the Search Query Performance report is another great way to find relevant negatives. The report shows all the various search queries that triggered your ads and generated a click. By reviewing these terms, you can identify any irrelevant keywords that trigger your ad and include them as negative keywords. You can manually add negative keywords to your keyword lists at both the ad group and campaign level. Simple add the irrelevant terms by placing a negative sign (–) before the word in your keyword list.
More info: Grants blog