Keep in mind that WCAG 2.0 is not an introduction to accessibility, but a reputable technical standard guideline created by the W3C to make web content more accessible to all people, especially to people who have to use assistive technology to access websites. The WCAG consists of twelve guidelines that are divided into four principles, namely Perceivable, Operable, Understandable, and Robust. Each of the four principles are measured against three levels--A, AA and AAA--of which A represents the lowest compliance to the web accessibility standard and AAA the highest.
The first principle from the Web Content Accessibility Guidelines, Perceivable, focuses on making content available and recognizable for all users, despite the user’s way of perceiving information or the tools they use to access this information; for example, screen readers. In short, content can’t be invisible to all of the user's senses. Here's an excerpt from WCAG 2.0:
"All web content...