The WCAG accessibility standard is divided into three levels: A, AA, and AAA. Broadly speaking, you can think of these as follows:
A – critical accessibility issues you must fix
AA – important accessibility issues you should fix
AAA – other accessibility issues that are desirable to fix
In many countries, public sector websites are required to meet AA. This applies to the UK, EU, and parts of America, like California.
Issues are generally harder to fix as you progress from A to AAA.
Full AAA compliance is exceedingly difficult and rare; even the W3C who created it doesn’t meet this standard. We recommend all websites aim to comply with AA.