A teaser is usually the beginning of a page, post, wiki, etc., and is usually displayed in a list with other teasers. Teasers can be trimmed to the first certain amount of characters, (e.g., 300 characters), or they can have a "teaser break" inserted in a place that makes sense. We used a "teaser break" in your first lesson.
A teaser is not a summary, but serves almost the same purpose. Teasers take almost no thought to implement, but it is recommended that your content have summaries, not teasers.