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"
God speaks of a person's desire for eternal life as if it were thirst and of that person receiving eternal life as if he were drinking life-giving water.