From Joomla! Documentation
This is an article about web banners. To learn about the Joomla! banner component, see our Banner help screen.
Impressions refer to the number of times the banner has been displayed, while click-throughs refer to the number of times that users have clicked on the displayed banners. The original content provider on whose page the banner was displayed receives a small payment for each impression and a more substantial payment for each click-through, which will often be used to pay for the costs of providing the content to the users.
If banners are too attention-grabbing, users often get annoyed with them and employ an ad-blocker, which prevents the display of banners.
More info on banners is available on Wikipedia.