When you make a search on Google underneath each blue link there are few lines of text (sometimes just one) taken from the page. Before linking to the page, you already know aproximately the context of the keywords in bold.
Before in certain cases, most of all specific searches - where long queries (more than three words) are involved -, the lines provided were not enough to give an accurate context. To solve it Google has added more lines to the snippets, so to give more context. This also enhances selective retrieval, cause there is the possibility of chosing the most relevant result among others similar.
