Microsummaries are regularly-updated succinct compilations of the most important information on web pages. For a brief overview of microsummaries, see this walkthrough of proposed microsummary functionality in Firefox.
For more information about microsummaries, check out the Microsummary proposal. If you are using a microsummary-enabled web browser, and you wish to test its microsummary-related functionality, see these unit tests or this list of microsummary generators for real-world web pages.