This option lets you display a short description of a post under the title on the Homepage and archive pages as illustrated below:
To add the excerpt to the post, simply write a couple of sentences in the Excerpt field under the post editor area. An excerpt can be as shot not as long as you would like.
Please note that this way is manual excerpt which will be shown under the title of the post. In case, a post has no manual excerpt, the WordPress will generate an excerpt automatically by selecting the first 55 words of the post.
Please check here for further info: https://codex.wordpress.org/Excerpt