What does -> mean in PHP?

What does -> mean in PHP?

What does it do and when should I use it?
Here is an example of its use:
public function __construct($server, $username, $password, $db)
{
$this->server = $server;
$this->username = $username;
$this->password = $password;
$this->db = $db;
$this->connect();
}
And used within WordPress:
<?php if($post->post_type == 'post') :?>
[ do stuff ]
<?php endif;?>