There are three conformance levels in the WCAG Guidelines, Level A (the lowest level), Level AA, and Level AAA (the highest level). These levels of conformance build on each other, adding additional success criteria at each level. Each level is progressively harder to achieve.
Level AA is the most commonly referenced accessibility standard, cited by most laws and regulations. Level AAA, the highest and most strict level of accessibility conformance, is typically considered above and beyond.
Some Level AAA standards are extremely strict and meeting them is not always realistic or possible for all types of content. Other AAA standards are relatively easy to implement and can have a broad, positive impact on user experience. The use of section headings to organize content (SC 2.4.10) and ensuring that link text is meaningful even out of context (SC 2.4.9) are both WCAG 2.1 AAA success criteria that are easy to implement, for example. Aiming to meet as many AAA success criteria as possible will help ensure your website or application is accessible to the largest number of people possible.
In general, level AA conformance is typically the goal. Level AA is usually the level required by accessibility laws. Think of Level AAA WCAG success criteria as the icing on the cake.