Impressions are exactly how many times your ad has been displayed for a search done for a specific keyword. Looking at the number of impressions is a good way of seeing just how many people are searching for services using that particular keyword. Impressions are known to fluctuate greatly depending on any number of variables that affect large numbers of people. News, weather, seasons, holidays all have been known to affect the amount of people searching at any given time. Though there are many people searching, that does not mean that you are showing up for every search. The number of times an ad shows up for a specific search is not guaranteed by the search engines, but it based mostly on your budget.