Quests, which include your Daily Quests and Milestones, are a way to earn rewards in the game by doing things you'd normally do.  They're also a great way to help new players learn how the game works.

Daily Quests are just what they sound like: Quests that you get to do daily! There are 10 Daily Quests in total. Completing and claiming a Daily Quest will give you a point of progress toward different Daily Quest rewards. If you complete 8 Daily Quests you’ll get all the rewards for that day. Make sure to check in and get as many as possible to maximize your loot!