What does "bindings" middleware do in Laravel 5.6?

Just as per the title. Default api middleware in Laravel 5.6 is listed in Kernel.php as:

protected $middlewareGroups = [
    'api' => [
        'throttle:60,1',
        'bindings',
    ],
];

I'd appreciate a layman's explanation of what bindings does, which I can't find anywhere in the docs or on the internet?

via Chebli Mohamed