This is the root of your WHMCS installation. Where this is, depends on what system you are using and where you are hosting.
The easiest way to find this though would be to look for the WHMCS directory structure.
The base (or root) WHMCS folder will look similar to the image shown below.
Simply find the folder in your installation which looks similar to this structure and that will be your 'base WHMCS folder'.