WCAG stands for Web Content Accessibility Guidelines. 2.1 is the current version. AA refers to the conformance level that is most commonly required and accepted, not only in the United States but across the globe.
WCAG 2.1 AA basically amounts to 50 things you can do to improve the accessibility of your website, or mobile app for that matter.
More info: https://www.essentialaccessibility.com/blog/ada-guidelines