According to the W3C, a system is accessible if it complies with its 4 principles: perceptible, operable, understandable and robust. If you want to know more about accessibility I recommend you this video..
Each of these principles has a set of guidelines. Each guideline has a set of success criteria. While the principles and guidelines are not evaluable, the success criteria are.
Each criterion has an associated level of conformity that can be: A, AA or AAA. The minimum level is A. To achieve a level A, the system must meet all level A compliance criteria or provide a compliant alternative version. To achieve a level of AA compliance, the system must comply with A and AA. And to achieve AAA you must comply with AA and A.
In this video I tell you more about the levels and some examples of criteria and levels of compliance.