Each panel has 35 tiles, each holding a different tasks to unlock them. Gray tiles contain hidden details. They must be turned blue first before to display the hint of their challenges. By completing the tasks and unraveling the tiles, the tiles beside them will turn blue. This means that the details for that tile has been revealed but the task is incomplete.
Completed challenges will display an image in place of the tile. There, you can review the task and the reward you got from completing the challenge. There are also some tiles that contain Hammers. these hammers can unlock the tiles instantly and let you earn the reward without doing the challenge at all. This is useful for very hard challenges that you can't seem to complete.
However, there are certain tiles that can't be cracked by Hammers. If that's the case, you don't have any choice but to complete them as they are intended: through normal gameplay. One thing to take note as well is that the Hammers are only usable within the panel they are acquired. If you complete the panel, the unused hammers will go to waste. That said, you save your hammer for the most difficult challenges or the ones you don't want to partake and use them instead.