It describes how to go about documenting processes, systems, people, locations, equipment, controls, risks and opportunities so that you have a reference of what makes the organization tick. There is also a chapter on how not to do it.
This is generally considered a very dry subject guaranteed to cure insomnia, demonstrated by the fact that there are only two known Enterprise Architecture Modelling jokes. Done properly, however, you will gain insights into your organisation that you never thought possible and acquire the vocabulary and context to hold your own with EA practitioners. You may even find the modelling exercise interesting.
The resulting consistent, unambiguous business model and lexicon can provide benefits beyond the obvious, such as a training manual, new employee orientation guide, reference manual, proof of compliance document and much, much more.