It seems that Pipfile / Pipfile.lock are intended to be replacements for requirements.txt, in the context of Python packaging. There isn't much documentation out there on how these actually work, however. I found an evolving description of pipfile on the PyPi section of the Python website here but it's pretty messy and doesn't explain the semantics of the different sections of the file.
Any pointers on how to understand these files?
As explained above by @Charles Duffy, it is a direct analog of Gemfile and Gemfile.lock from the Ruby world. See reference below for more details.
Reference: https://medium.com/never-hop-on-the-bandwagon/gemfile-and-gemfile-lock-in-ruby-65adc918b856