Excerpt is a short text which can be displayed on post archive pages or in some widgets instead of the full post content. You can add excerpt for standard posts, knowledge base articles and FAQ posts.
If you are using the WordPress block editor (Gutenberg), you can add excerpt in the right panel when editing a post (make sure you are switched to Document tab at the top of the panel). If you are not seeing the Excerpt field, click on “dots” icon in upper right corner to enable the field.
If you are NOT using the block editor (for example when you are using the Classic Editor plugin), then while editing a post, click on Screen Options in upper right corner, check Excerpt and scroll down to see the Excerpt field under the editor.