A Turn represents a time-frame in which you may perform actions in Combat.
At the beginning of each Turn, your AP and MP values restore to their maximum amount (though this may be reduced, or enhanced, by other effects).
You then have 30 seconds in which you decide what to do, whether its to Move your character, to cast Spells, to Attack others, or simply pass your turn. When this time ends, the next player's turn starts.
It's favourble to end your turn fast to prevent getting a bad Speed Bonus.