Daily goals are special tasks generated for players every 12 hours. These goals are optional, players are not required to reach them, but they may want to complete them to get a reward.
Daily Goal consists of 4 sub-tasks of various kinds: killing monsters, upgrading items, completing collections, mixing recipes in alchemy, etc. To receive a reward players should complete any 3 of 4 sub-tasks, the last sub-task can be completed to receive an additional reward.
Sub-tasks are being checked for completeness whenever you visit your home page. As soon as you complete the required amount of sub-tasks you can collect the reward. It is only possible to collect a reward once per goal so if you've completed 3 of 4 sub-tasks and collected a reward you won't be able to finish the last sub-task and collect the remaining part of a reward. You should decide whether you want to collect a part of a reward or try to complete goal in full to get increased reward.
If you didn't reach a goal in time or didn't collect a reward - it is being automatically deleted and replaced with a new goal.