Collection Events are challenges that task players with obtaining units of specific items during their levels as a secondary objective to win amazing prizes!
When the Collection Event is live, every level you win where you collect units of the required item, for example, blue pieces, will count towards your Challenge progress until your next prize is awarded and you're assigned the next objective.
After completing all tasks, there'll be a Grand Prize! But don't snooze, or else you might find yourself scrambling during the last minutes of the event to complete as many levels as possible before the time runs out.