Installation describes how to install CGAL, and lists the third party libraries on which CGAL depends, or for which CGAL provides interfaces.
Organization of the Manual gives an idea where you should look for documentation. The documentation for a class, may be spread over manual pages of base classes, and reference manual pages of concepts the class is a model of.
Preliminaries lists the licenses under which the CGAL datastructures and algorithms are distributed, how to control inlining, thread safety, code deprecation, checking of pre- and postconditions, and how to alter the failure behavior.