Contextual links allow the user to perform actions directly on a particular entity or other portion of the site. Adding one is done by creating a {module}.links.contextual.yml file in the root directory of your module. In order to add a contextual link for the mastering_drupal_8 module, it would look like:

mastering_drupal_8:
 title: 'Test'
 route_name: 'mastering_drupal_8.test_page_block'
 group: 'block'  

For each contextual link element, you have the following attributes:

• title: Title of the link in the contextual link area
• route_name: Machine name of the route that the contextual link directs to
• group: Area the contextual link appears in for example, "block," "menu," and so on
• weight: Determines the order of the contextual links; higher weight links appear later

The contextual link route will receive information about the entity or other item that the contextual link was triggered on. For example, if our route is mastering_drupal_8/test/{block}, it would be injected with the block name to be passed to our controller.