This statement resides in the URL helper which is loaded in the following way:
$this->load->helper('url');
The redirect function loads a local URI specified in the first parameter of the function call and built using the options specified in your config file.
The second parameter allows the developer to use different HTTP commands to perform the redirect "location" or "refresh".
According to the Code Igniter documentation: "Location is faster, but on Windows servers it can sometimes be a problem."
Example:
if ($user_logged_in === FALSE)
{
redirect('/account/login', 'refresh');
}