Let's say you want to create nested variables, where the "bar" variable is a child of the "foo" variable.

---
- hosts: localhost
  vars:
    main:
      foo: "hello"
  tasks:
  - name: output the main[foo] nested variable
    debug:
      var: main[foo]

  - name: output the main['foo'] nested variable
    debug:
      var: main['foo']
---

Running this playbook should return the following. This shows that the nested variable must be wrapped in single quotes to return the value of the nested variable.

TASK [output the main[foo] nested variable] 
ok: [localhost] => {
    "main[foo]": "VARIABLE IS NOT DEFINED!"
}

TASK [output the main['foo'] nested variable] 
ok: [localhost] => {
    "main['foo']": "hello"
}

Let's consider a slightly different situation, where one of the nested variables is localhost. In this scenario, instead of using main['localhost']['foo'] you could use main[inventory_hostname]['foo'].

---
- hosts: localhost
  vars:
    main:
      localhost:
        foo: "hello"
  tasks:
  - name: output the value of the foo nested variable
    debug:
      var: main[inventory_hostname]['foo']
---

If inventory_hostname resolves to localhost, then the following output should be returned.

TASK [output the value of the foo nested variable] 
ok: [localhost] => {
    "main[inventory_hostname]['foo']": "hello"
}