What does double colon in laravel means

You are likely encountering this as a way of accessing a static method or property of a class.

For example:

class Foo
{
    public static function bar()
    {
      return "bar";
    }
}

Foo::bar // access the bar method without instantiating the Foo class.

:: Scope Resolution Operator

:: is called as scope resolution operator (AKA Paamayim Nekudotayim). This operator is used to refer the scope of some block or program context like classes, objects, namespace and etc. For this reference an identifier is used with this operator to access or reproduce the code inside that scope.

Reference

Auth::guard($guard)->guest() : In this line you are using the guard() method of static class Auth. To use the function of a static class we use :: Scope Resolution Operator.