Every day, Travian Kingdoms offers mini-quests that you can complete to earn rewards.
To view these daily quests, open the Quest book by clicking the corresponding icon on the left side of your village view: 
then select the "Daily quests" tab.
There are three quest slots in your book. Each can contain a task. This means you can store a maximum of 3 daily quests at a time. Click on the quest name to view the task.
Once you've completed a quest, you can collect your reward, and the slot will be cleared. A new quest will appear after a short time with a countdown showing the exact moment.
If you're not interested in a specific quest or don't think you can complete it, you can exchange it for a new task by clicking the icon at the bottom of the screen: