For most Web pages listed on a search engine results page (SERP), the Web page’s approximately 160-character meta description tag is what appears directly below the page title.
The meta description is less important for the actual ranking of the page and much more important as the page synopsis and call to action that trigger the clickthrough.
Although you cannot A/B test meta descriptions in real time, since the SERP is of course beyond your direct control, you can experiment with different meta descriptions in series and monitor clickthrough rate on the Traffic Sources > Search Engine Optimization > Queries report. (If you have not linked your SEO data from Google Webmaster Tools to Google Analytics, you can check clickthrough data directly in your Google Webmaster Tools reports.)
An increase in organic clickthrough rate (CTR) for a specific keyword, with no increase in average position, probably indicates a more compelling meta description.
As a note, since page title is much more important than meta description in search engine ranking, it’s much riskier to experiment with page title to increase clickthrough rates.