Info documentation is essentially a form of online (meaning on the computer, not necessarily on the network) documentation, very similar to help files in many Windows applications. The largest similarity is that the different documents in the Info system are linked to each other. These links are very much like hyperlinks on the web.
UNIX, of course, already has a system of online documentation in man pages, but there are two reasons for knowing about and using GNU Info. Firstly, the Free Software Foundation's GNU software often does not have up to date man pages these days, and the man pages that do exist usually contain a warning to look at the Info pages instead. Secondly, man pages for large pieces of software are very unpleasant. They are single documents and you sometimes need to search for a very long time to find what you want. The many cross-referenced small documents in Info are often easier and faster to use.
The best way to get started is to do the Info tutorial. Although
there is a standalone info reader, this tutorial is much better to do
inside emacs. If you start up emacs, pressing Ctrl-h will
bring up a message at the bottom of the screen saying something
like
C-h (Type ? for further options)-
If you then type
i you will be taken to the Info directory in emacs, which is
an index of all available Info pages. At the top of the directory are
a few instructions, one of which is that typing h will take
you to a tutorial. Do this, and learn how to use Info. Once you are
familiar with the system, it will probably be faster to use the
standalone reader which is just called `info', rather than emacs.