It’s usually displayed on archive pages in conjuction with your post. The excerpt is entered in a meta field labeled ”Excerpt”, which is displayed close to the bottom of the screen when you’re editing a post. If you can’t see the field, scroll to the top of the page, click the Screen Options button and make sure that ”Excerpt” is checked in the ”Boxes” group. If you haven’t written a manual excerpt for your post, and your active theme displays excerpts as part of its design, WordPress will generate an automatic excerpt from the first paragraphs of your post content and show that instead.
Only posts support excerpts out of the box in WordPress, but themes and plugins can add excerpt support to pages and custom post types. The RSS feed of a WordPress site can be set to only display the excerpt for posts, rather than the full content, by going to Settings → Reading, finding the ”For each article in a feed, show” setting and selecting ”Summary”.