This guide is for keeping a consistency and code manageability in the open source projects developed by Google using C++.  As explained in the guide:

C++ is the main development language used by many of Google’s open-source projects. As every C++ programmer knows, the language has many powerful features, but this power brings with it complexity, which in turn can make code more bug-prone and harder to read and maintain.

The goal of this guide is to manage this complexity by describing in detail the dos and don’ts of writing C++ code. These rules exist to keep the code base manageable while still allowing coders to use C++ language features productively.

This guide also serves the purpose of making the beginners understand the language’s conventions. Read the full guide here.