Is "/private" just an ordinary directory or does it have special properties
There is no magic at works here, this will just create a user-writable directory to store the module in. You can‘t copy it into /usr/libexec/apache2 as this is protected by SIP. 

Using /private like this seems kind of odd though. I would probably create /usr/local/libexec/apache2 and install the module there. You just need to be careful to adapt all references to it as well.