https://www.w3.org/WAI/WCAG21/Understanding/

W3C

This resource provides a detailed technical description of WCAG 2.1 guidelines and  Success Criteria used to evaluate accessibility and interpret VPATs.

Abstract

“Understanding WCAG 2.1” is a guide to understanding and using Web Content Accessibility Guidelines (WCAG) 2.1. It is part of a set of documents that support WCAG 2.1. Please note that the contents of this document are informative (they provide guidance), and not normative (they do not set requirements for conforming to WCAG 2.1). See Web Content Accessibility Guidelines (WCAG) Overview for an introduction to WCAG, supporting technical documents, and educational material.