A more thorough explanation:

An Index of Patents is a yearly publication by the U.S. Patent and Trademark Office. It is divided into two parts: Part I contains a list of all the patentees for the year, while Part II is an index of the subjects of inventions. The index is arranged by class and subclass, making it easy to find information on a particular invention.

Let's say you are interested in finding information on a new type of solar panel. You could use the Index of Patents to search for patents related to solar panels. You would look in Part II, under the appropriate class and subclass for solar panels, and find a list of all the patents related to that subject. This would give you valuable information on the latest developments in solar panel technology.