A teaser is a short piece of text, usually the first paragraph
or two of an article. This text is then displayed in most lists,
including the default home page (/node). Many modules (including Views)
are designed to work with node teasers, and the concept of a "teaser"
vs the node "body" is integral to the workings of Drupal core and the Drupal UI.
The teaser can have image but it is not mandatory.
Teasers can have one or two buttons (of type flat) which are in the actionbar.
Content hierarchy is achieved by the order of fields and the use of typography.
Try to limit the amount of information and actions in a teaser.
On mobile the image field is not displayed. On tablet and wider
the maximum height of the teaser should match the heigth of the image_style.