Excerpt is a small summary of the full post or page content that can show your readers a brief overview of what they can expect when reading the full page.
To set a custom excerpt, go to the Post or Page screen and find the Excerpt box. If you do not see it below the text editor or in the sidebar then click on Screen Options button at the top right corner of the screen. Look for the Excerpt checkbox and ensure it is checked. Close the Screen Options tab and the Excerpt box will be shown below the text editor.
Write your custom excerpt inside the Excerpt box. When ready, publish your post or page. Please note that the excerpt feature only works on certain pages.