Mercurial is a cross-platform version control system. It was designed to work with big projects and large amounts of code, which undoubtedly are present in the OpenJDK project. The OpenJDK official repository is a Mercurial.
The Forest plugin is the one used for various OpenJDK subprojects to merge and coexist. It works with nested Mercurial repositories, which normally are regarded as isolated. The main idea is to propagate changes from the root repository to the nested ones.
The main purpose of it is to allow a developer to work with the code, which is a minor part of a full OpenJDK project repository, without needing to make any changes to the whole repository (change a revision number, for example).
First of all, we will need to install Mercurial itself. On Windows it can be done by going to the official Mercurial site and downloading it from http://mercurial.selenic.com/wiki/Download.
For Linux distributions, there are, usually, Mercurial versions...