How to define a class and in which folder to define a custome validation rule in laravel 5.2

How to define a class and in which folder to define a custome validation rule in laravel 5.2. In their doc for validation they have given the following code .

Validator::extend('foo', 'FooValidator@validate');

But they haven't mentioned what is this FooValidator and if it is a class name or file name , then where where to place this file. I need help on this .

via Chebli Mohamed