'Impressions' can mean a few different things depending on the context of how it is used. One of the more common uses relates to search impressions where a site, news post or webpage will appear in a Google Search Listing. Over a period of time there will be a number of impressions where they appear & a % of people will click on the link to visit the site. So a site might have 10,000 impressions and an average click rate of 1% or 1,000 people visiting its pages. Individual stories or articles might have a 20% click rate if they are seen to be relevant or interesting. Seeing impressions information in isolation is of little use without seeing the corresponding click rates.