What it means for your advertising
Impressions tell you how many times your ad had a chance to be seen. On their own they say nothing about whether anyone cared, so read them alongside clicks and conversions.
How an impression is measured varies by channel. A search ad counts when it appears on the results page; a social ad may need to be on screen for a moment first. Treat it as the basic unit of advertising, roughly one display of your ad.
Some channels charge per thousand impressions. That's CPM.
An example
Java Roasters' office coffee ad on Google Search was shown 12,000 times in two weeks: 12,000 impressions, of which 240 led to a click.
Java Roasters is a fictional business, and its figures are illustrative.
How we handle it
Impressions are the first figure in your Performance Hub, with a delivery graph showing how they build day by day. If they're low, budget, targeting or bids may be holding the campaign back, and finding which is our job. More on Google Search advertising.