Python is an interpreted language, similar to Perl in that respect. Or you can think of it as a super duper shell script.
If you have a program called "hellworld.py" you can run it like so:
python helloworld.py
And that's that.
In terms of pylrs, it's a library for converting e-books. It's not a final product, so
pylrs itself will not convert books for you. You'd need to write a program that utilizes the pylrs library.
The pylrs library itself comes with some usage examples, so if you want to write a whole program, that's where to start. From a programmer's perspective, it's pretty straight-forward. But if you're not a programmer, this isn't what you're looking for.
And I get the impression that you are, indeed, looking for a finished product for conversion. I am aware of two which utilize pylrs for conversion, and they are fully finished products.
pielrf - A text to LRF (Sony EBook Format) converter. It mimics the look of books you download from the Sony Connect Store, and it's really easy to add chapters, headers, and Tables of Contents. Give you one guess why I know about this one!
https://www.mobileread.com/forums/showthread.php?t=10752
libprs500 - Don't let the name deceive you, it's not ONLY a library -- it has competed programs to do conversion for you as well, plus a number of other features, like transferring LRFs to the Reader via USB. Here's the link to the html2lrf (part of libprs500) discussion:
https://www.mobileread.com/forums/showthread.php?t=10582
If you want a finished product, those are the way to go.
-Pie