Daily quests are quests available to players who reached level 25 and higher. You can get them from the NPC Foreman (present in all cities but Thereall) or from the Activity Billboard everyday, and will reward you with both Gold and Green Coupons you can exchange for various prizes from the Curiosities Dealer. They also give a lot of experience points, and that's why they are one of the main ways to level up fast.
These quests are the same everyday, but they change everytime you level up 5 times, until you reach the level cap.