To add an excerpt to a post, simply write one in the Excerpt field under the post edit box. An excerpt can be as short or as long as you wish. Usually, given its purpose, a couple of sentences is fine.
Interestingly, since the WordPress excerpt is similar in purpose to the META description of (X)HTML documents, excerpts can additionally be used as meta descriptions too. Some themes do this by default. It can also be done by means of an SEO plugin or a plugin for managing data in the head of (X)HTML pages.