What Is An Impression?
Generally speaking, an impression is the number of times a link URL appears in the search results (not clicked upon)
The link does not necessarily have to be scrolled into view, or for that matter, be visible to count as an impression.
What Is a Search Result Position?A position is calculated from top to bottom on the primary side of the search results page, then top to bottom on the secondary side of the search results page.
The above image displays positioning. Take note; images have position 2 and Knowledge Panel cards and other types of result features such as Google business places on the right side of the page carry position 11, even though they are in a more prominent position they carry less ‘weight’.
If business listings were to appear where the images now appear, the business listing would then carry position 2 and not 11.
What Counts As A Position Placeholder?Only elements containing a link to a website count as a position. For example; images which point to new Google searches are not counted as position placeholders.
Always remembering that some image thumbnails in the main search page do (eventually) lead to a web page, so they are counted as a position placeholder. And if the link or image does not lead to a site outside of Google, it is not counted as a placeholder.
How To Calculate Position?If a query returned your page at positions 3, 6, and 9 on the front page, its position is counted as the topmost position, which is 3. If a second query (let us say someone from a different country using a different browser searches the same query) finds your page at positions 1, 4, and 7 on the front page, your average page position is then counted as 1.
The average position across these two queries can then be calculated by adding the two topmost figures, then divide them by two (3 + 1)/2 = 2
Furthermore. A link must get an impression (appear in the search) for its position to be recorded. If your page does not appear for the query, it gains no position. And if the result is on page 4 of the search results, and the user only views page 1, then its position is not recorded for that query.
What Is A Click?A click sends the user to a page outside of Google Search. Clicking a link that stays inside Google search results is not counted as a click.
If you click on a link within the search results like an image that does not navigate outside of Google you basically perform a new query, this is called a query refinement. If a link is a query refinement link, clicks and impressions are not counted for that link they are only counted as a click when you navigate outside of Google (to a website)
For more details navigating to Google What are impressions, position, and clicks article.