The debug module can be used to print output.  In this example, "Hello World" will be printed.

- debug: 
    msg: "Hello World"

When the play is run, the following should be produced.

TASK [debug]
ok: [server1.example.com] => {
    "msg": "Hello World"
}

Often, debug is used along with the register module. In this example, the stdout/stderr/rc of the ls (list) command will be stored in a variable named "out". 

- name: list command
  shell: "ls /tmp"
  register: out

You can then printing the output using the debug module, like this.

- name: standard out
  debug: 
    msg: "{{ out.stdout_lines }}"

- name: standard error
  debug: 
    msg: "{{ out.stderr_lines }}"

- name: return code
  debug: 
    msg: "{{ out.rc }}"

Running this playbook should return the following. In this example, standard out returns a file or directory in the /tmp directory named "foo", there was no standard error, and the return code is 0 (success).

TASK [list command]
ok: [server1.example.com]

TASK [standard out]
ok: [server1.example.com] => {
    "msg": "foo"
}

TASK [standard error]
ok: [server1.example.com] => {
    "msg": ""
}

TASK [return code]
ok: [server1.example.com] => {
    "msg": "0"
}