I heard this term used to manage the code base on GitHub. What does it mean?
As stated in the GitHub docs :
The Fork and Pull model allows anyone to deploy an existing repository and make changes to their personal plug without requiring access to the source repository. Then the changes should be pulled into the source repository by the project developer. This model reduces the amount of friction for new entrants and is popular in open source projects because it allows people to work independently without prior coordination. [...] Pull queries are especially useful in the Fork and Pull models because they provide a way to notify accompanying projects of changes in your fork.
GitHub consists of many repositories. You can branch out any repository and start developing changes. When you finish your changes, you can send a pull request to the author of the repository where you branched from to merge your changes into the main repository. If the author of the repository discovers that your changes are significant, the transfer request can be accepted and your changes combined.