It is the number of times the recommendations widget is loaded on your front end. As an example, if you use the "Similar products" recommendations, which is usually on the product page, it will be the number of product views for the month. This is because each time the product is viewed, it will load the widget 1 time.
If the widget has 8/12/24 product suggestions, these are NOT counted towards the impressions count. Only the number of times the recommendation widget was loaded.