What does my php mean?

I'm writing up my ICT coursework and basically need to give a description of what everything means. I copied some coding off of the internet and would love if someone could explain what this section of PHP means in theory so I can understand?

    <?php 
      print "<h2 align=\"center\">Login to staff only page</h2>";
    // If password is valid let the user get access
    if (isset($_POST["password"]) && ($_POST["password"]=="$password")) {
    ?>