first is a Jinja2 filter, used to get the first value in an array (also known as a list). The last filter can be used to return the last value in an array. Let's say you are using the vars plugin to create an array of fruit, like this.

vars:
  fruit: 
    - apple
    - banana
    - orange
    - grapes

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

- name: output the contents of the 'fruit' array
  debug: 
    msg: "{{ fruit }}"

Which should return the following.

TASK [output the contents of the 'fruit' array]
ok: [server1.example.com] => {
    "msg": [
        "apple"
        "banana",
        "orange",
        "grapes"
    ]
}

The first filter can be used to get the first value in the list, like this.

- name: output the first value in the 'fruit' array
  debug: 
    msg: "{{ fruit | first }}"

Which should return the following.

TASK [output the first value in the 'fruit' array]
ok: [server1.example.com] => {
    "msg": [
        "apple"
    ]
}