These are first and last letters of the Greek alphabet. Possible meanings are 1) "the one who began all things and who ends all things" or 2) "the one who has always lived and who always will live." Alternate translation: "the A and the Z" or "the first and the last"
Possible meanings are 1) "the one who began all things and the one who will cause all things to end" or 2) "the one who existed before all things and who will exist after all things."