These are generally used to store messages that will be displayed to the user. For example if you have a method that tries to create a user and it fails, you can get the relevant you can you may have the following in a User Controller:
$create = $this->create_user($user);
if($create)
{
// user created
$message = Kohana::message('user', 'create_success');
}
else
{
// failed to create user
$message = Kohana::message('user', 'create_error');
}
This is used for configuration information such as the hash_method used in auth module and you can access it using Kohana::$config->load('auth.hash_method')