A caption (also known as a summary) is the area of a Search Engine Results Page (SERP) devoted to a particular result. Captions help users to decide which result to click on and help site owners to communicate to users what to expect on their page. Captions are an oft-forgotten tool of the SEO toolkit.
Ranking is certainly the most important factor in determining user clicks, but captions are also a contributor. This post, give you a primer on captions, and you all of the components of a caption and give a bit of insight about how Bing generates each of those components.
Every search result has a caption, which consists of four parts: the title, the snippet, the URL, and the Preview: