In Drupal we can display secondary menu as tabs. If you don't know what it is open any content item (node) as administrator and on the top you can see:

• View 
• Edit
• Delete

and so on

In this post we will review how to add a static local task

In order to have a new tab you need to create a new YML file in the root of your module. This file has a specific name your_module_name.links.task.yml. You have to replace your_module_name with the name of your module

Then you can define the local task you want to provide in the next format  

your_module.admin: # Unique id of your tasks normally prefixed with route name or entity name 
  route_name: your_module.custom_settings # Route which will be opened, when we click on the tab
  title: 'Settings' #Title of your tab
  base_route: entity.node.canonical #Parent route (the default tab) 
  parent_id: entity.node.canonical # Optional: We define the parent tab id here
  weight: 100 # Defines the order for the tab

Let's review a simple example. I have module local_tasks. You can find it here  https://github.com/LOBsTerr/drupal-modules-examples/tree/master/local_tasks or create your own.

I have a custom page, which will provide the extra settings for nodes, in fact it will just display the title of a node for a sake of simplicity. This a route for which we are going to add a tab. You need to add it to local_tasks.routing.yml

local_tasks.node_extra_settings:
  path: '/node/{node}/extra-settings'
  defaults:
    _controller: '\Drupal\local_tasks\Controller\LocalTasksController::nodeExtraSettings'
    _title: 'Node extra settings'
  requirements:
    _permission: 'access content'
  options:
    parameters:
      node:
        type: 'entity:node'

The definition of the tab you need to add to local_tasks.links.task.yml

# In this example we add an additional tab to the node page
local_tasks.node_extra_settings: # The ID of local task is the same as route, to make it easier to control.
  route_name: 'local_tasks.node_extra_settings' # The name of our route, provided in local_tasks.routing.yml.
  title: 'Extra settings' # This string will be displayed as a title of a tab.
  base_route: 'entity.node.canonical' # We use node entity in order to group tabs together.
  weight: 100

Now, you need install the module local_tasks or to clean the cache if you do it your own module.

Open any node (node/[NID]), you should see a new tab "Extra settings". Also, you can open directly the page node/[NID]/extra-settings

In the next posts, we will review how to add sublevels of tabs, to define the default local task and how to add local tasks dynamically