The GNU Project distributes most of its on-line manuals in the Info format, which you read using an Info reader. You are probably using an Info reader to read this now.
There are two primary Info readers:
info, a stand-alone program
designed just to read Info files (see Stand-alone Info), and the
info package in GNU Emacs, a
general-purpose editor. At present, only the Emacs reader supports
using a mouse.