What is middleware in Laravel?

php artisan make:middleware CheckAge

class CheckAge {
    public function handle($request, Closure $next) {
        if ($request->age <= 18) {
            return redirect("home");
        }
        return $next($request);
    }
}

// app/Http/Kernel.php
protected $routeMiddleware = [
    "age" => AppHttpMiddlewareCheckAge::class,
];

Route::get("admin", function () {
    //
})->middleware("age");