From the official blog post:

(The average position column) provides a quick at-a-glance way to see where your site is showing in the search results for specific queries. The other change youll notice is that were showing a "Displaying" number for Impressions and Clicks. This number represents a total count of the data displayed in the Search Queries table. The number in bold appearing just above it is a total count of all queries including the "long tail" of queries which are not displayed in the Search Queries table. When the "Displaying" number is not visible, such as when you select a specific country from the "All countries" drop-down menu, then the bold number is the total count of the data displayed in the Search Queries table.

The image below shows both of these changes.

The other significant addition is the "starred" tab which lets webmaster put a star next to their most important search queries. Google says that "all of the queries that you "star" will be consolidated under the Starred tab providing a super easy way to view just the queries you care about."

Google is asking for feedback from webmasters about these changes to the report, so feel free to head over to the Webmaster Help Forumand tell them what you think.