A Post Impression is how many times the Post is loaded within a content block on the pages of your site.
A Widget Impression is the number of times a Widget (content block) is loaded on a page.
The Widget Impression is primarily used to determine a Click Through Rate (CTR).
Each time a content block loads on one of your pages, a set number of posts are also loaded. (The default is 12.) Not all of the posts are visible because of the depth of the widget. But they still count as having an impressions, just as banner ads do even if they are "below the fold" on a web site.
CTR is calculated by taking the number of times a Widget (content block) is loaded divided by the number of clicks an Advertiser gets on its Posts when they appear in that content block.