An impression is counted every time your ad, post, or search listing is shown to a person. It does not matter whether they clicked, read it, or even consciously noticed it — if it appeared on their screen, that is one impression.
For example, if a dentist in Augusta runs a search ad and it appears at the top of the results 2,000 times in a week, that is 2,000 impressions. Whether anyone clicked is a separate question answered by click-through rate.
Impressions are a top-of-funnel metric. They tell you how much visibility you are getting, which is the raw material everything else is built from — you cannot get clicks or leads from an ad no one sees.
