Library is a collection of SVG files each containing one drawing of a component. The library structure follows KiCAD library structure - each footprint (module) is a separate file placed in directories representing libraries.
It is also possible to have multiple libraries with different component style.
When specifying multiple module libraries, the first library path to match a
given footprint is used for rendering. The lookup order is the same you
wrote the --libs option.
All the details about the library can be found in its repository. Note that the library is essential for this script and unfortunately it is still incomplete - contributions are welcomed! Drawing a single component from scratch takes less than 10 minutes, which is not much time. Please, send a pull-request for components you have created.