max is a Jinja2 filter, used to return the largest value in an array (also known as a list). The min filter is used to return the lowest value.Let's say you are using the vars plugin to create an array of integers.

vars:
  integers: 
    - 2
    - 4
    - 3
    - 1

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

- name: output the contents of the 'integers' array
  debug: 
    var: integers

Which should return the following.

TASK [output the contents of the 'integers' array]
ok: [server1.example.com] => {
    "integers": [
        "2"
        "4",
        "3",
        "1"
    ]
}

The max filter can be used to get the largest value from the array.

- name: output the max value in the 'integers' array
  debug: 
    var: integers | max

Which should return the following.

TASK [output the max value in the 'integers' array]
ok: [server1.example.com] => {
    "integers | max": "4"
}