The -e or --extra-vars command line option can be used to create variables on the command line, like this.

ansible-playbook playbook.yml --extra-vars "foo=Hello bar=World"

In a playbook, the debug module can be used to print each variable.

---
- hosts: all
  tasks:
    - debug:
        msg: "{{ foo }}"

    - debug:
        msg: "{{ bar }}"

Something like this should be returned.

TASK [debug]
ok: [server1.example.com] => {
    "msg": "Hello"
}

TASK [debug]
ok: [server1.example.com] => {
    "msg": "World"
}

Precedence

The -e or --extra-vars command line option will take precedence over every other way that a variable can be set. For example, let's say you attempt to update the foo variable to contain a value of "Goodbye" using the vars plugin.

---
- hosts: all
  vars:
    foo: Goodbye
  tasks:
    - debug:
        msg: "{{ foo }}"

The foo variable will retain it's value of "Hello" because the -e or --extra-vars command line options take precedence.

TASK [debug]
ok: [server1.example.com] => {
    "msg": "Hello"
}