Each time a user enters a search query into Google that triggers your ad to be shown is known as an impression. The higher your budget is the more impressions you are likely to receive, this is because Google will not want to show your ad at every opportunity as it will use up your budget too quickly. Google must allow for budgeting so that there's enough budget left for a click on each impression.
You can refine or expand the number of impressions that you receive by altering the keyword matching options that you use, if you use tighter matching options then you are likely to receive less impressions. In comparison, if you do not use any keyword matching options then your ads may show to users with irrelevant search queries. You can also gain more impressions by displaying your ad groups in the content network.
The statistics in Adwords display the number of impressions your account receives each day and which keywords/ ad groups they are coming from.
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