Why truthy value should be one of [false, true]?

I am struggling with proper YAML formatting. I have my Ansible playbook:

---
- hosts: all
  become: yes
  gather_facts: true
  tasks:
  - dnf: name=chrony state=present
  - service: name=chronyd state=started enabled=yes

and have run yamllint on it

  3:11      warning  truthy value should be one of [false, true]  (truthy)
  6:3       error    wrong indentation: expected at least 3  (indentation)
  7:52      error    no new line character at the end of file  (new-line-at-end-of-file)

What does truthy value mean?

Regarding your initial question and description, because of an syntax error and the incorrect indention. It should be

---
- hosts: all
  become: true
  gather_facts: true
  
  tasks:

  - name: Ensure chrony (for time synchronization) is installed.
    yum:
      name: chrony
      state: present

  - name: Ensure chrony is running
    service:  
...

It will also resolve the message about

3:11 warning truthy value should be one of [false, true] (truthy)

as become: yes is changed to become: true.

For the answer to the "why" you may have a look into specifying the string value 'yes' in a YAML property.