问题:

I have heard this term used for managing a code base in GitHub. What does it mean?

回答1:

As noted within the GitHub docs:

The Fork & Pull Model lets anyone fork an existing repository and push changes to their personal fork without requiring access be granted to the source repository. The changes must then be pulled into the source repository by the project maintainer. This model reduces the amount of friction for new contributors and is popular with open source projects because it allows people to work independently without upfront coordination. [...] Pull requests are especially useful in the Fork & Pull Model because they provide a way to notify project maintainers about changes in your fork.

回答2:

GitHub consist of many repositories. You can fork any a repository and start developing your changes on that. When you are completed with your changes you can send pull request to author of repository from where you forked to merge your changes to main repository. If repository author find your changes significant, the pull request can be accepted and your changes are merged.