Impressions are views to your mobile ads. You will see a breakdown of "Impressions" and "People Reach" in your daily reports for each mobile ad.  "People Reach" are the number of individual people saw your ad.  Your ad will be shown to the same person more than once (approx no more than 5 times every 24 hours).  This helps build awareness about your business. This is why your "Impressions" number will always be higher than your "People Reach".