Clicks: Counted when an affiliate link or marketing asset (ie. a banner) is clicked on, and a user/customer lands on your site.
Impressions: Counted when an image banner shared by your affiliates has been 'seen' (i.e. loaded up from our servers).
Conversions: Counted when an affiliate has successfully driven a conversion (a sale, signup, or a lead depending on what it is you're tracking).
The number of tracking events per plan vary. If you go over your plans allocated number of clicks, views, or conversions, these will be billed as an overage.