Impressions are the number of times a particular content is displayed.

Each time this post shows up in your feed, it will raise the Impression number for this post by one. You don’t even have to click on it. It doesn’t even matter if you are the only person to see it, and you see it five times.

It’s a number that shows that your content is being displayed in various feeds. It does not show the number unique visitors or the number of visitors that actually clicked on the content.