WCAG guidelines are organized into three successive level of compliance: A, AA, and AAA. AAA is the most stringently compliant and difficult to obtain. Each successive level is progressively harder to meet, and each level of conformance indicates having met all of the criteria for the previous levels as well. There are 25 criteria which you must meet to reach level A. For level AA, you must meet 13 additional requirements. Level AAA requires compliance with 23 additional criteria.
If your website meets level A, the majority of users will be able to use your website. Each additional level and criterion met makes your site accessible to more and more people. The level you should aim for as a business is AA, which is the level that’s legally required for certain types of sites, and the one that is typically referred to when you’re tasked with “making a website accessible/ADA compliant.”