This is the root of your Joomla 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 Joomla directory structure.
The base (or root) Joomla 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 Joomla folder”.
