echo $_SESSION["username"], what does it mean code example

Example 1: create session in php

<?php 
  // Start the session
  session_start();

  // Set session variables
  $_SESSION["color"]= "blue";
  $_SESSION["animal"]= "dog";
  echo "The session variable are set up.";
?>

Example 2: php session variables

<?php
   session_start();
   $_SESSION['var'];
?>