What do impressions in the Chrome extension developer dashboard mean?

The impressions are the number of times the extension has appeared in search (both the Web store search and Google). The user does not have to visit the page to register an impression and if the user goes directly to the page via a link in an email etc. it will not count as an impression. This is why my impressions are less than my installations (people send links on Facebook and twitter a lot more than they search for it).

Impressions are the number of times your extension is displayed to users.

When the store wall is populated with items, you get an "impression" when your extension's tile is scrolled into view on the screen. This results in one "impression" per page load (in other words, you don't get an impression every time your item is scrolled into view, then out, then back in again).

Impressions are the number of times your app appears on the screen of someone browsing the app store, but there is more to it. If you app is scrolled through on the screen, you get one impression. If the user scrolls back up and your app reappears on the screen, nothing is counted (since the would be double counting).

If a user clicks on the app plate while browsing the app store, you get a second impression. For example, imagine yourself browsing the app store and scrolling down on the page. Every app that populates your screen view is getting an impression. Let's assume that you realize that you just saw something interesting and you scroll back up so that the app of interest repopulates the screen. This app (or any other app) does not get a second impression from your action. But now, you click on the app to see it in full screen view. This action triggers a second impression.

Now if you install the app, then, obviously, this action counts as an install. So in summary, per session, each app can get at most 2 impressions, and a single install.