Negative Keywords are words or phrases that prevent your ad from appearing on Amazon's search engine results page when those terms are in a Search Term (the customer's search query).
Negative keywords can be added at the campaign or ad group level.
Example: adding "pick" as a negative phrase will eliminate "guitar pick," "tooth pick," "pick," and "the quick brown fox."
Example: adding "pick" as a negative exact will only eliminate "pick," and not the multiple word variations.