A manual excerpt is the real summery for an article because it does not automatically pull content from anywhere. You must write out a manual excerpt yourself.
When creating/Editing a new post, there’s an “Excerpt” meta box available at the bottom for doing just that and here it is.
If you do not see it, it is most likely hidden by default. Scroll up to the top of the post edit screen and click the “Screen Options” drop-down. In there, make sure the “Excerpt” box is checked.