It is the sum total of the number of page views, where the recommendations widget is loaded by the API. Hence for Similar products, which are usually on the product page, it will be the number of product views for the month, cause each time a product is viewed, it also loads the "Similar products" widget on the page.

If a widget has 8 product thumbnails, these are NOT counted towards the impressions count. Only the number of times the recommendation widget was loaded.