lower is a Jinja2 filter, used to update a string to all lower case characters. The upper filter is used for all upper case characters. In this example, "Hello World" will be filtered to lower case.

---
- hosts: all
  tasks:
  - name: output 'Hello World' filtered to lower case
    ansible.builtin.debug: 
      msg: "{{ 'Hello World' | lower }}"
...      

Which should return the following.

TASK [output 'Hello World' filtered to lower case]
ok: [server1.example.com] => {
    "msg": [
        "hello world"
    ]
}

Let's say you are using the vars plugin to create a variable. The debug module can be used to output the variable, like this.

---
- hosts: all
  vars:
    foo: "Hello World"
  tasks:
  - name: output the 'foo' variable
    ansible.builtin.debug: 
      msg: "{{ foo }}"
...          

Which should return the following.

TASK [output the 'foo' variable]
ok: [server1.example.com] => {
    "msg": [
        "Hello World"
    ]
}

The lower filter can be used to update the variable to all lowercase characters. Notice that foo is not wrapped in quotes, so that foo is interpreted as a variable, not a string.

---
- hosts: all
  vars:
    foo: "Hello World"
  tasks:
  - name: output the 'foo' variable, lower case
    ansible.builtin.debug: 
      msg: "{{ foo | lower }}"
...      

Which should return the following.

TASK [output the 'foo' variable, lower case]
ok: [server1.example.com] => {
    "msg": [
        "hello world"
    ]
}