The reverse shows a wreath with the letters VOT V MULT X which translates as "Votis quinqennalia multis decennalia" which means : Because of the vows, five years, through more vows, ten years. This derives from the Roman custom of offering vows of a certain sacrifice to the Gods in return for the granting of some favor or wish. At the beginning of his reign, the emperor would offer vows to have a prosperous five year term and when these five years would have been reached he would make the sacrifices in fulfillment of those vows and would offer vows for another five years. The vows were a money sacrifice paid to the gods (later and in Theodosius' case the Catholic Church).
(note that they only paid on delivery of the goods )