By default, most tasks are run on your managed nodes (the target systems) and some are run on the control node (that's your Ansible server).Refer to these articles for a better understanding of how tasks are run against the control node or managed nodes.

• Run tasks on the control node
• Run tasks on your managed nodes

delegate_to can be used to run a module on a specific host. In this example, delegate_to is used with the file module to remove a file on locahost.

---
- hosts: all
  tasks:
  - name: remove foo.txt on control node
      module: file
      path: /tmp/foo.txt
      state: absent
      delegate_to: localhost
...

In this example, delegate_to is used to run the task on server1.example.com.

---
- hosts: all
  tasks:
  - name: remove foo.txt on control node
      module: file
      path: /tmp/foo.txt
      state: absent
      delegate_to: server1.example.com
...

Often, the run_once parameter is used so that the task is only ran one time on the control node.

---
- hosts: all
  tasks:
  - name: remove foo.txt on control node
      module: file
      path: /tmp/foo.txt
      state: absent
      delegate_to: localhost
      run_once: true
...

Running this task should produce the following.

TASK [remove foo.txt on control node] 
changed: [server1.example.com -> localhost]

Let's say you have two hosts in your default hosts file or your own inventory file.

all:
  hosts:
    server1.example.com:
    server2.example.com:

And you want to pass a variable from server1 to server2. For example, let's say /tmp/example.txt on server1 contains "Hello World".

In this example, the shell module is used to read /tmp/example.txt on server1 and the register parameter is store Hello World in the out variable. The set_fact module is then used to create a variable named greeting and delegate_to and delegate_facts are used set the greeting variable as a fact on server2. The debug module is then used to see that the greeting variable has been set on server2.

---
- hosts: all
  tasks:
  - shell: cat /tmp/example.txt
    register: out
    when: inventory_hostname == 'server1.example.com'

  - set_fact:
      greeting: "{{ out.stdout }}"
    delegate_to: server2.example.com
    delegate_facts: true

  - debug:
      var: greeting
    when: inventory_hostname == 'server2.example.com'
...

Running this playbook should produce the following.

TASK [shell]
changed: [server1.example.com]
skipping: [server2.example.com]

TASK [docker : set_fact] 
ok: [server1.example.com -> server2.example.com]
skipping: [server2.example.com]

TASK [docker : debug]
skipping: [server1.example.com]
ok: [server2.example.com] => {
    "greeting": "Hello World"
}