When flying from Jakarta to Tulum, the time of day you decide to fly out makes little to no difference towards the overall price you’ll pay on tickets.
Great question - CGK to CUN just means Jakarta Soekarno-Hatta Intl Airport to Cancun Airport. Now you’ll know what these unique codes when you look for flights! Cozumel Airport (CZM), Tulum Intl Airport (TQO) are other airports near this flight route and their unique identifiers/IATA codes.