Events are features that are available for a limited time. They are optional.
Some events require the players who participate to prepare a certain number of things before it ends. Completing the orders is only possible through completing match-3 episodes. If you complete a match-3 level at first try, you get to prepare more things. If you reach the goal in time, you get the big event reward. But there are also rewards available along the way.
You can find detailed info about an event under its own article.