join is a Jinja2 filter that can be used to convert a list into a string. Let's say you are using the vars plugin to create an list of fruit. The debug module can be used to output the entire list, like this.

---
- hosts: localhost
  gather_facts: false
  vars:
    fruit:
      - apple
      - banana
      - orange
      - grapes
  tasks:
  - name: debug fruit
    debug:
      var: fruit
...

Which should return the following which shows that "fruit" is a list.

TASK [debug]
ok: [localhost] => {
    "fruit": [
        "apple", 
        "banana", 
        "orange", 
        "grapes"
    ]
}

The join filter can be used to convert the list into a string.

---
- hosts: localhost
  gather_facts: false
  vars:
    fruit:
      - apple
      - banana
      - orange
      - grapes
  tasks:
  - name: debug fruit
    debug:
      var: fruit | join
...

Which should return the following.

ok: [localhost] => {
    "fruit | join": "applebananaorangegrapes"
}

Arguments can be passed into join. In this example, a single white space will be placed between each element.

- debug:
    var: fruit | join(' ')

Which should return the following.

ok: [localhost] => {
    "fruit | join(' ')": "apple banana orange grapes"
}

Or, as a bit of a more practial example, here is how you could append /tmp/ before a list of files.

---
- hosts: localhost
  gather_facts: false
  vars:
    files:
      - foo.txt
      - bar.txt
      - hello.txt
      - world.txt
  tasks:
  - debug:
      msg: "/tmp/{{ files | join(' /tmp/') }}"
...

Which should return the following.

ok: [localhost] => {
    "msg": "/tmp/foo.txt /tmp/bar.txt /tmp/hello.txt /tmp/world.txt"
}

And here is how you could use a variable in the join filter.

---
- hosts: localhost
  gather_facts: false
  vars:
    files:
      - foo.txt
      - bar.txt
      - hello.txt
      - world.txt
  tasks:
  - set_fact:
      base_directory: "/usr/local/files/"

  - debug:
      msg: "{{ base_directory }}{{ files | join(' ' + base_directory + '') }}"
...

Which should print the following.

ok: [localhost] => {
    "msg": "/usr/local/files/foo.txt /usr/local/files/bar.txt /usr/local/files/hello.txt /usr/local/files/world.txt"
}