What is meant by a partially-initialized module and fully-initialized module? I searched for it on Google but couldn't find anything related to it.
It's exactly what it says in plain English: the module's initialization is incomplete.  This is in reference to an error message, typically for a missing symbol definition.
The reason behind this is that you imported a file, which starts the initialization process.  During this process, a required symbol proved to be undefined, almost always due to an unintended circular reference.
A simple search for +partially defined Python module will bring you plenty of use cases.  I hope the above explanation bridges your gap in understanding the issue.
What is meant by a partially-initialized module and fully-initialized module?