To display an article summary you have to use the Excerpt field. In most cases this field is not activated by default. To activate it follow the steps bellow:
Activate the Excerpt field in the Classic editor
- Open or create a Post
- Click on Screen Options (from the upper right hand side of the screen) and Enable Excerpt Field
Activate the Excerpt field in the Gutenberg editor
- Open or create a Post
- Click on Settings Options (from the upper right hand side of the screen)
- Then, click on Options and activate the Excerpt field from the modal window