Jelly and Jenkins
The config.jelly
configuration file is a simple Jelly template. You can find out more about Jelly in Jenkins in the following link:
https://wiki.jenkins-ci.org/display/JENKINS/Basic+guide+to+Jelly+usage+in+Jenkins
You can read more about Jelly, in general, here at http://commons.apache.org/proper/commons-jelly/. This article states the following:
Jelly is a Java and XML based scripting and processing engine.
The main purpose of Jelly in this context is to provide developers with a highly flexible tag library through which they can quickly and easily create and handle UI view changes.
From a developer point of view, Jelly files interact with Java code to get and set declared values at runtime and presents them to the user via the UI.