The register parameter can be used to store the standard output (stdout), standard error (stderr) and return code (rc) of a command. In this example, the stdout/stderr/rc of the ps (show processes) command will be stored in a variable named "out". 

- name: ps command
  shell: ps
  register: out

You can then do something with the register, such as printing the output using the debug module.

- name: standard out
  debug: 
    var: out.stdout_lines

- name: standard error
  debug: 
    var: out.stderr_lines

- name: return code
  debug: 
    var: out.rc

Running this playbook should return something like this.

TASK [standard out]
ok: [server1.example.com] => {
    "msg": "4012  pts/2  00:00:00  bash"
}

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

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