This is the best description of GD&T I’ve yet to read:
What is GD&T?
GD&T, the abbreviation for Geometric Dimensioning and Tolerancing is a set of standardized symbols and conventions that are used to describe parts in a way that makes it easier for customers, manufacturers, and other supply chain participants to successfully communicate. Parts that are manufactured in a shop must meet specific specifications. These specifications are shown on engineering drawings, which are normally produced by CAD software. GD&T is a particular set of conventions used on engineering drawings…that communicate how parts should fit together and how they function. This course will teach you the basics of how to understand GD&T symbols and their use…