For questions about creating either a private or competing version of an existing open project.
"Forking" is the process of creating either a private version of an open source project, or creating a competing version.
A classic example of the latter is OpenOffice vs LibreOffice.
In software engineering, a project fork happens when developers take a copy of source code from one software package and start independent development on it, creating a distinct and separate piece of software. The term often implies not merely a development branch, but a split in the developer community, a form of schism. -- wikipedia
The word "fork" is used to reflect that such an event often represents a "fork in a road" where an existing project begins to take two (often diverging) development paths.