In simple terms, WCAG 2 Conformance Level A is less strict than Level AA — and is therefore more “easily” obtainable and maintainable. Level A conformance means all WCAG 2 Level A requirements (success criteria) have been satisfied. Level AA conformance satisifed all the WCAG 2 Level A requirements, in addition to the Level AA requirements. You can read more information about understanding conformance requirements, or view the quick reference guide to WCAG 2 requirements.