After you've been running your keywords for some time and have accumulated impressions on individual keywords, you may wish to pause or delete the low performing terms. You can also edit your poor performers to make them more specific, such as by changing their match type or by refining the search term with additional phrases.
First, analyze the current list of keywords to determine which keywords you may want to remove or edit. Following are the keywords to pay special attention to:
Keywords with low CTR. Low CTR means that users are not finding your ads relevant to the search.
Low Quality Score keywords. You'll need to add this column to your reports through the Customize columns button in your Keywords tab. Optimize keywords that have Quality Scores below 4.
Keywords that are costing a lot but are not generating any sales.
Keywords that have a high cost per conversion.