capitalize is a Jinja2 filter, used to update a string so that the first character is upper case and all subsequent characters are lower case. Or, the title filter could be used as well. In this example, "hello world" will be updated to "Hello world".

- name: "output 'hello world', upper case first character"
  debug: 
    msg: "{{ 'hello world' | capitalize }}"

Which should return the following.

TASK [output 'hello world', upper case first character]
ok: [server1.example.com] => {
    "msg": [
        "Hello world"
    ]
}

Let's say you are using the vars plugin to create a variable, like this.

vars:
  foo: "hello world"

The debug module can be used to output the variable, like this.

- name: "output the contents of the 'foo' variable"
  debug: 
    msg: "{{ foo }}"

Which should return the following.

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

The capitalize filter can be used to update the variable so that the first character is upper case and all subsequent characters are lower case. Notice that foo is not wrapped in quotes, so that foo is interpreted as a variable, not a string.

- name: "output the contents of the 'foo' variable, upper case first character"
  debug: 
    msg: "{{ foo | capitalize }}"

Which should return the following.

TASK [output the contents of the 'foo' variable, upper case first character]
ok: [server1.example.com] => {
    "msg": [
        "Hello world"
    ]
}