Many themes display a list of recent blog posts or articles, showing only a few lines of text from each — a teaser. Depending upon the theme, this may be automatic; the first 55 words of the post (an Automatic Excerpt) are used. If that’s the case with your theme, you have the option to use a Custom Excerpt, by filling in the Excerpt field. If, instead, your theme shows full posts, you’ll need to use the More Tag. In the Visual Editor, just place your cursor and then click the More Tag button to insert it where you want the teaser to end.