WordPress Excerpt is a summary of an article, of a page, or of a portfolio. This field is not displayed as a default in the post edit screen. To active it, click the Screen Options at the very upper right corner of your dashboard, and enable the Excerpt option. Based on your layouts option, the Excerpt is used to add a specific summary to your thumbnails, or add a summary to your blog preview.
Here are a few examples of Excerpt:
- Excerpt texts in use: http://www.undsgn.com/uncode/blog/blog-full-width-grid/
Once you have inserted the Excerpt you need to set the thumbnails blog preview to use this text in the Posts → Module or you can set to use the Full Text and set the chars number option, screenshot.