A, AA, and AAA compliance in web accessibility are levels of conformance to the Web Content Accessibility Guidelines (WCAG) established by the World Wide Web Consortium (W3C). These guidelines provide a set of standards that web developers can use to make their websites and web applications more accessible to people with disabilities.
A-compliance is the minimum level of conformance and it meets the basic accessibility requirements for most users with disabilities. It includes requirements such as providing text alternatives for images, making sure that text and background colors have enough contrast, and providing a way for users to skip repetitive navigation.
AA-compliance is considered a higher level of conformance and it addresses a wider range of accessibility needs. It includes all of the requirements of A-compliance, and additional requirements such as providing text alternatives for non-text content, providing keyboard-only navigation, and providing captions or subtitles for audio and video content.
AAA-compliance is the highest level of conformance and it addresses the needs of users with the most severe disabilities. It includes all of the requirements of A and AA compliance, as well as additional requirements such as providing a way to extend the time for users to complete timed activities, providing a way for users to adjust the font size, providing sign language or other forms of alternative text for audio content.
It's worth noting that A, AA, and AAA compliance are not strict pass/fail criteria, but rather a set of guidelines that can be met to a certain degree. Also, the compliance level that should be met depends on the context of the website, the target audience and the resources available to make it more accessible.