The @ sign in Laravel web.php route files

What exactly the '@' sign stand for in laravel route file web.php
Ex: Route::get('/', 'HomeController@index');? please don't tell me what this code does I already know that. but the question is why the @ sign??
The'@' sign is not a method but the index is. so what is name of this @ sign and what exactly it does? I'm just curious to know why??? the @ sign.

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