The accessor and mutator both are the public functions of Laravel model for getting and setting model's attributes.
An accessor will automatically be called by Eloquent when attempting to retrieve the value of the first_name attribute:
$fullName = $user [...]
Go ahead and look at the contents of the config/app.php file. You'll find an array entry that lists all the service providers that will be loaded during the bootstrapping of the Laravel application (with core a [...]