The Search impact category in the +1 Metrics of Google webmaster tools, shows how +1's from Google+ effect impressions and clicks. Its separated from the main statistics.
Keep in mind that you might not see any data here, due to the Google+ button still being very new. According to Google, it won't be statistically significant until your pages have thousands of +1 impressions. This can take some time, since people are only just starting to use Google+.
Search impact is similar to the information found in the Top Pages tab of Search queries, only it counts the data from +1's alone.
The +1 annotated clicks represents the number of times someone clicked a page, which was shown in the result.
The +1 annotated impressions represents how many times someone saw pages in the results, due to someone in their network having submitted the page to Google+.
All Impressions includes both the impressions from +1's and the impressions without. The same goes for the All Clicks option.