The initials represent the formats for recording (first initial), mixing (middle initial), and transfer (last initial). "A" is short for analog and "D" is for digital. CDs are DDD, completely digital, ADD, analog recording with digital mixing and transfer, or AAD, analog recording and mixing with digital transfer to CD.
None of these formats inherently results in superior sound quality. It's probably best to ignore these codes when chosing CDs.