For the longest time, I totally ignored this field, I confess. I didn’t see the point, really. When it was finally explained to me what this field is, though? Dude, you better believe I hopped on that sucker.
Simply put: the meta description is the field of text that search engines display when showing that page in search results.
Ignore this field at your peril.
A creative, short description — especially one that works in a keyword or two in a sensible, organic way — can boost your rankings for that page (see above: my post is the first result for that search).
Remember to keep it under 150 characters, as most search engines will only display that much text.
So use your meta description field! (By the way, Thesis makes this SO freaking easy.) (Yes, that’s an affiliate link.) Make it descriptive, make it funny, make it compelling, but also make it relevant, always keeping in mind your readers. It will not only increase your SEO but also serve as an additional enticement (together with your awesome headline, of course) to the searcher to actually click the link and read your content.