[0] can be used to get the first character from the string "Hello World". 

- name: get the first character from string 'Hello World'
  debug: 
    msg: "{{ 'Hello World'[0] }}"

Which should return the following.

TASK [get the first character from string 'Hello World']
ok: [server1.example.com] => {
    "msg": [
        "H"
    ]
}

Or to get the first character from a variable.

- name: get the first character from variable 'foo'
  debug: 
    msg: "{{ foo[0] }}"

[-1] can be used to get the last character.

- name: get the last character from string 'Hello World'
  debug: 
    msg: "{{ 'Hello World'[-1] }}"

[:3] can be used to get the first 3 characters.

- name: get the first 3 characters from string 'Hello World'
  debug: 
    msg: "{{ 'Hello World'[:3] }}"

[-3:] can be used to get the last 3 characters.

- name: get the last 3 characters from string 'Hello World'
  debug: 
    msg: "{{ 'Hello World'[-3:] }}"

[:-3] can be used to get everything except for the last 3 characters.

- name: get everything except for the last 3 characters from string 'Hello World'
  debug: 
    msg: "{{ 'Hello World'[:-3] }}"

[3:] can be used to get everything except for the first 3 characters.

- name: get everything except for the first 3 characters from string 'Hello World'
  debug: 
    msg: "{{ 'Hello World'[3:] }}"

You may also want to use the trim filter to remove whitespace from the left and right sides of a string.

- name: output 'Hello World', trim whitespace
  debug: 
    msg: "{{ '  Hello World  ' | trim }}"

Which should return the following.

TASK [output 'Hello World', trim whitespace]
ok: [server1.example.com] => {
    "msg": [
        "Hello World"
    ]
}