What is a fork?
Explanation of the term ► Fork English translation: Bifurcation
A Fork (falsely called Hack) is an event that sometimes appears in the development of a computer project, typically in community projects (case of many free software)
For Guppy, a Fork says when you modify a part of code to customize it.
Unlike a Plug-in, which is the development of an independent script with a "clean" code that is added to the initial project without altering it "thus guaranteeing a version upgrade without any history..."