It is the password field which you create by editing the visibility of the page and then click on “Password protected”.
It’s in fact only this:
<form action=”http://www.pagename.com/wp-login.php?action=postpass” class=”post-password-form” method=”post”>
<input name=”post_password” id=”pwbox-6648″ type=”password” size=”20″>
<input type=”submit” name=”Submit” value=”Enter”>
</form>